Add __pychache__ to .gitignore

Summary:
When running shell_building, the tree gets littered with __pycache__ files,
hence ignore them.

Reviewed By: wez

Differential Revision: D14664303

fbshipit-source-id: d41837cdfda1ae93c2d9330f00284128b3f7f7d0

build/fbcode_builder/.gitignore

 # Facebook-internal CI builds don't have write permission outside of the
 # source tree, so we install all projects into this directory.
 /facebook_ci
+__pycache__/