V4L/DVB (4105): Remove uneeded prototype

Uneeded extern prototype is removed, as the function is 
prototyped at media/v4l2-common.h.

Signed-off-by: Trent Piepho <xyzzy@speakeasy.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>