Missing python_callbacks.h in sdist build

[Kota Yamaguchi]

sdist package does not bundle python_callbacks.h required for building a wheel package.

There is an issue that the source tree has hard-coded #include <faiss/python/python_callbacks.h>, that should be located inside the C++ header location. One option is to apply a patch to rewrite this to #include "python_callbacks.h" and bundle the source in the source package.