editorconfig: Set indent_size for BUCK files to 4 (#21554)

Summary:
The `BUCK` build files' indentation are 4 spaces large. This PR adds a small modification to `.editorconfig` to reflect that. Especially important for them, because an improperly indented line would be a syntax error.
Pull Request resolved: https://github.com/facebook/react-native/pull/21554

Differential Revision: D10247954

Pulled By: TheSavior

fbshipit-source-id: 98596a563e9f8c36060cd8aa702757eff230c4c3