[media] V4L: add missing EXPORT_SYMBOL* statements to vb2

videobuf2-memops and videobuf2-core can be compiled as modules, in which
case 3 more symbols from videobuf2-memops.c have to be exported.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>