Add project.ignore to .buckconfig to reduce watched files (#3468)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/3468

Some CI jobs can fail with "OS file watch limit reached" when running
buck2. This section should reduce the number of files that it tries to
watch.

imported-using-ghimport

Test Plan: Imported from OSS

Reviewed By: larryliu0820

Differential Revision: D56857491

Pulled By: dbort

fbshipit-source-id: ce3a39f27e57ee5318028a58924e317fcf4f34d4