V4L/DVB (6324): fix videobuf_cgmbuf export

As videobuf_cgmbuf is defined only if CONFIG_VIDEO_V4L1_COMPAT is enabled,
move the EXPORT_SYMBOL_GPL declaration inside the #ifdef block.  Fixes
compilation for x86_64 defconfig.

Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>