BLD: Add missing inline declarations
I am compiling pandas with libpython into a static binary. It works after this patch; otherwise the symbols in the headers result in redefinition errors, since they are included in both `tslib.pyx` and `parser.pyx`. (This issue does not affect regular builds of pandas because tslib and parser are compiled and loaded as separate dynamic libraries.) `numpy_helper.h` already annotates all other symbols defined in it with `PANDAS_INLINE`, so I though that this patch is straightforward and did not explain this creating the pull request.
 ## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14611?src=pr) is 85.27% (diff: 100%)

> Merging [#14611](https://codecov.io/gh/pandas-dev/pandas/pull/14611?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will not change coverage

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  Hits          43230      43230          
  Misses         7463       7463          
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```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [2e276fb...1df4eb3](https://codecov.io/gh/pandas-dev/pandas/compare/2e276fb2fec6dd04d6abcf5e79c03853ee86cd24...1df4eb3a056ff81a7214c24e05af200b0f48dc39?src=pr)
 The latest patch :
- adds missing inlines
- prepares `total_seconds` and `get_long_attr` in `datetime_helper.h` for removal (now only `ujson/python/objToJSON.c` depends on them)
- fixes import error reporting that I saw [here](https://ci.appveyor.com/project/jreback/pandas-465/build/1.0.1766/job/hscus0h1el92sq0h) (`ImportError: C extension: l_seconds not built`)
- declares `cmp_pandas_datetimestruct` in `np_datetime.h` which is used by `tslib.pyx` through the cimport in `datetime.pxd`
 can you rebase / update. I think this got lost in the shuffle. can you rebase? I accidentally deleted my branch before pushing, and now that I pushed the rebased patch I have no authority to reopen the issue. @orivej Even core contributors don't have this authority :-) so you can just open an new PR