MAINTAINERS: add another Android binder maintainer

As briefly discussed with Greg and Todd, add Christian as maintainer for
binder{fs}.
Signed-off-by: NChristian Brauner <christian@brauner.io>
Acked-By: NMartijn Coenen <maco@android.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

MAINTAINERS

@@ -950,6 +950,7 @@ M:	Arve Hjønnevåg <arve@android.com>
 M:	Todd Kjos <tkjos@android.com>
 M:	Martijn Coenen <maco@android.com>
 M:	Joel Fernandes <joel@joelfernandes.org>
+M:	Christian Brauner <christian@brauner.io>
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/staging.git
 L:	devel@driverdev.osuosl.org
 S:	Supported