mpc512x kernel configurations without SPI support do not build:

drivers/media/video/fsl-viu.c: In function 'viu_open':
drivers/media/video/fsl-viu.c:1248: error: implicit declaration of function 'kzalloc'
drivers/media/video/fsl-viu.c:1248: warning: assignment makes pointer from integer without a cast
drivers/media/video/fsl-viu.c: In function 'viu_release':
drivers/media/video/fsl-viu.c:1335: error: implicit declaration of function 'kfree'

If CONFIG_SPI is enabled, the slab.h will be included in
linux/spi/spi.h which is included by media/v4l2-common.h
and the fsl_viu.c driver builds.

Let's incluce linux/slab.h directly to fix the build breakage.
Signed-off-by: NAnatolij Gustschin <agust@denx.de>
Signed-off-by: NMauro Carvalho Chehab <mchehab@redhat.com>