Include vec256 headers in setup.py (#17220)
Summary:
Fix #16650.
Headers such as `ATen/cpu/vml.h` contain `#include <ATen/cpu/vec256/vec256.h>`
for example, but these vec256 headers aren't included, due to commit e4c0bb1.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/17220
Differential Revision: D14165695
Pulled By: ezyang
fbshipit-source-id: 27b2aa2a734b3719ca4af0565f79623b64b2620f
