Add Meta internal config file to gitignore (facebook#13307)

Summary:
As title

Pull Request resolved: facebook#13307

Test Plan: CI

Reviewed By: cbi42

Differential Revision: D68289968

Pulled By: jaykorean

fbshipit-source-id: ddcdcb57ad53479f322b2492279d7eec74bb624d

Author: jaykorean

.gitignore

@@ -91,6 +91,7 @@ buckifier/__pycache__
 .arcconfig
 
 compile_commands.json
+meta_gen_cpp_compile_commands.json
 clang-format-diff.py
 .py3/