Fix CMakeLists.txt for Int8 python bindings (#15047)

Fix CMakeLists.txt for Int8 python bindings (#15047)

Summary:
Currently in caffe2, one cannot properly fetch the content of Int8 blobs.

Upon digging the source code, it turns out that the relevant source code is not being compiled. Adding the source to CMakeLists.txt fixes this issue.

First time ever doing a pull request. Please let me know if there's any rule I should follow. Thanks.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15047

Differential Revision: D13417583

Pulled By: bddppq

fbshipit-source-id: dd39575971a3012635edbf97a045d80e4b62a8eb