tty: linux/gsmmux.h needs linux/types.h

We use __u8 in linux/gsmmux.h, so include linux/types.h to have that
defined.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>