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>

drivers/media/video/cx88/cx88.h

@@ -302,7 +302,6 @@ struct cx88_dmaqueue {
 	struct btcx_riscmem    stopper;
 	u32                    count;
 };
-struct cx88_core;
 
 struct cx88_core {
 	struct list_head           devlist;