Add react-native/build to .gitignore

Summary:
This folder should have been added to the .gitignore.
Gradle is placing some build file over there in the latest version. This prevents it.

Changelog:
[Internal] [Changed] -

bypass-github-export-checks

Reviewed By: cipolleschi

Differential Revision: D74884379

fbshipit-source-id: 7da9682589c2bc161c1b0e3d741d1049b94505e4