MAINTAINERS: add another Android binder maintainer

As briefly discussed with Greg and Todd, add Christian as maintainer for
binder{fs}.

Signed-off-by: Christian Brauner <christian@brauner.io>
Acked-By: Martijn Coenen <maco@android.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>