media: v4l: omap_vout: vrfb: include linux/slab.h

Without this header, we get a build error in some configurations:

drivers/media/platform/omap/omap_vout_vrfb.c: In function 'omap_vout_setup_vrfb_bufs':
drivers/media/platform/omap/omap_vout_vrfb.c:143:26: error: implicit declaration of function 'kzalloc'; did you mean 'vzalloc'? [-Werror=implicit-function-declaration]

Fixes: 6a1560ec ("media: v4l: omap_vout: vrfb: Convert to dmaengine")

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>