Explicitly define cmp_pandas_datetimestruct
Fixes build warning
```
pandas/_libs/tslib.c:78967:17: warning: implicit declaration of function 'cmp_pandas_datetimestruct' is invalid in C99 [-Wimplicit-function-declaration]
```

lgtm. ping on green. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17750?src=pr&el=h1) Report
> Merging [#17750](https://codecov.io/gh/pandas-dev/pandas/pull/17750?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/97fea48945163ec27349918c14ec9bdca2335460?src=pr&el=desc) will **decrease** coverage by `0.04%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17750/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/17750?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17750      +/-   ##
==========================================
- Coverage   91.25%   91.21%   -0.05%     
==========================================
  Files         163      163              
  Lines       49856    49856              
==========================================
- Hits        45496    45474      -22     
- Misses       4360     4382      +22
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.01% <ø> (-0.03%)` | :arrow_down: |
| #single | `40.25% <ø> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17750?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17750?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/17750?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `63.38% <0%> (-1.82%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17750?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.73% <0%> (-0.1%)` | :arrow_down: |
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/17750?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `95.48% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17750?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17750?src=pr&el=footer). Last update [97fea48...0b9fff3](https://codecov.io/gh/pandas-dev/pandas/pull/17750?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17750?src=pr&el=h1) Report
> Merging [#17750](https://codecov.io/gh/pandas-dev/pandas/pull/17750?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/97fea48945163ec27349918c14ec9bdca2335460?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17750/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/17750?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17750      +/-   ##
==========================================
- Coverage   91.25%   91.24%   -0.02%     
==========================================
  Files         163      163              
  Lines       49856    49869      +13     
==========================================
+ Hits        45496    45502       +6     
- Misses       4360     4367       +7
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.04% <ø> (ø)` | :arrow_up: |
| #single | `40.24% <ø> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17750?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17750?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17750?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.73% <0%> (-0.1%)` | :arrow_down: |
| [pandas/plotting/\_style.py](https://codecov.io/gh/pandas-dev/pandas/pull/17750?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19zdHlsZS5weQ==) | `74.28% <0%> (ø)` | :arrow_up: |
| [pandas/core/internals.py](https://codecov.io/gh/pandas-dev/pandas/pull/17750?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW50ZXJuYWxzLnB5) | `94.38% <0%> (ø)` | :arrow_up: |
| [pandas/core/indexing.py](https://codecov.io/gh/pandas-dev/pandas/pull/17750?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhpbmcucHk=) | `93.97% <0%> (+0.03%)` | :arrow_up: |
| [pandas/io/formats/excel.py](https://codecov.io/gh/pandas-dev/pandas/pull/17750?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvZXhjZWwucHk=) | `96.69% <0%> (+0.04%)` | :arrow_up: |
| [pandas/core/indexes/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/17750?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9iYXNlLnB5) | `96.46% <0%> (+0.11%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17750?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17750?src=pr&el=footer). Last update [97fea48...991dbfe](https://codecov.io/gh/pandas-dev/pandas/pull/17750?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 this is causing some more warnings have a look at the travis logs Are you referring to 

```
In file included from pandas/_libs/period.c:250:0:
pandas/_libs/src/datetime/np_datetime.h:106:5: note: expected ‘struct pandas_datetimestruct *’ but argument is of type ‘const struct pandas_datetimestruct *’
 int cmp_pandas_datetimestruct(pandas_datetimestruct *a,
     ^
pandas/_libs/period.c:20547:3: warning: passing argument 2 of ‘cmp_pandas_datetimestruct’ discards ‘const’ qualifier from pointer target type [enabled by default]
```

?  AFAICT there's not much to be done about that unless we want to un-declare the `const` in np_datetime.c. so how does this PR help then? ```
the warning currently s

pandas/_libs/tslib.c:81135:3: warning: implicit declaration of function ‘cmp_pandas_datetimestruct’ [-Wimplicit-function-declaration]
   __pyx_t_2 = ((cmp_pandas_datetimestruct(__pyx_v_dts, (&_NS_MIN_DTS)) == -1L) != 0);
   ^
pandas/_libs/tslib.c: In function ‘__pyx_pymod_exec_tslib’:
```
 you need to declare ``const *`` the structure args in .h and change in .c thanks!