Update gitignore
Update gitignore

Summary: Include temporary gradle files and external cmake builds.

Reviewed By: IanChilds

Differential Revision: D4993374

fbshipit-source-id: e7f295b7c465a1f7ec640485a5e85a746eb524be

facebook/litho,facebook/litho,facebook/litho,facebook/litho,facebook/litho,facebook/litho