tty: move linux/gsmmux.h to uapi

linux/gsmmux.h defines a user interface and therefore should be
installed with other headers.

Make the file include:
* linux/if.h for IFNAMSIZ
* linux/ioctl.h for _IO* macros

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>