V4L/DVB (10792): cx88: remove unnecessary forward declaration of cx88_core

A recent patch added a forward declaration of cx88_core right before the
main definition of that structure, which is obviously unneeded.

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