Add a license file for klib (khash)
File contents are exactly those of: https://raw.githubusercontent.com/attractivechaos/klib/9a063b33efd841fcc42d4b9f68cb78bb528bf75b/LICENSE.txt

- [ ] closes #xxxx (Replace xxxx with the Github issue number) **Not applicable**
- [ ] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature **Not applicable**
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [ ] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature. **Not applicable**

thanks @musicinmybrain 