[media] videobuf2: correct the #ifndef text mistake in videobuf2-dma-contig.h

It should be a mistake due to copy & paste in header file
  Correct it in videobuf2-dma-config.h for avoiding duplicate include it

Change-Id: I1f71fcec2889c033c7db380c58d9a1369c5afb35

Signed-off-by: Albert Wang <twang13@marvell.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>