V4L/DVB: videobuf: Don't export videobuf_(vmalloc|pages)_to_sg

Those functions are only called inside videobuf-dma-sg.c, make them
static.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>