Update .gitignore for protobuf open source build
Update .gitignore for protobuf open source build

Summary: The C++ generated files from .proto shouldn't need to be checked in, so ignore them.

Reviewed By: yuxuanchen1997

Differential Revision: D30029275

fbshipit-source-id: 5c9aa52c1febc6619868c2e155e3cb2d15824bb5

facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex