[media] dvb-usb-dibusb-mc-common: Add MODULE_LICENSE

dvb-usb-dibusb-mc-common is licensed under GPLv2, and if we don't say
so then it won't even load since it needs a GPL-only symbol.

Fixes: e91455a1 ("[media] dvb-usb: split out common parts of dibusb")

Reported-by: Dominique Dumont <dod@debian.org>
Cc: stable@vger.kernel.org # 4.9+
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>