[media] DocBook: add the new videobuf2-v4l2 header

This header should also be used to generate documentation. So,
add it.
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>

Documentation/DocBook/device-drivers.tmpl

@@ -231,6 +231,7 @@ X!Isound/sound_firmware.c
 !Iinclude/media/v4l2-of.h
 !Iinclude/media/v4l2-subdev.h
 !Iinclude/media/videobuf2-core.h
+!Iinclude/media/videobuf2-v4l2.h
 !Iinclude/media/videobuf2-memops.h
      </sect1>
      <sect1><title>Digital TV (DVB) devices</title>