Input: add SW_MUTE_DEVICE switch definition

Some devices, such as new Intuos series tablets, have a hardware switch to
turn touch data on/off. To report the state, SW_MUTE_DEVICE is added
in include/uapi/linux/input.h.

Reviewed_by: Chris Bagwell <chris@cnpbagwell.com>
Acked-by: Peter Hutterer <peter.hutterer@who-t.net>
Tested-by: Jason Gerecke <killertofu@gmail.com>
Signed-off-by: Ping Cheng <pingc@wacom.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>