Let desktop SDL builds see the fullscreen option in the GameSettingsScreen, too.

There doesn't seem to be any reason why they can't, especially when there's a message that can be sent to toggle it in the first place.

I'm not sure what the issue creator in https://github.com/hrydgard/ppsspp/issues/5953 meant by "fullscreen button is missing", but this should resolve the obvious lack of fullscreen-ness.