[JIT] Export JIT backend extension headers in setup.py (#38525)
Summary:
**Summary**
This commit adds the headers required to define and use JIT backends to
`package_data` in `setup.py` so that they are exported and copied to the
same place as the rest of the headers when PyTorch is installed.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/38525
Differential Revision: D21601806
Pulled By: SplitInfinity
fbshipit-source-id: 1615dd4047777926e013d7dd14fe427d5ffb8b70
