V4L/DVB: v4l: fsl-viu.c: add slab.h include to fix compile breakage

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: Anatolij Gustschin <agust@denx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

drivers/media/video/fsl-viu.c

@@ -22,6 +22,7 @@
 #include <linux/interrupt.h>
 #include <linux/io.h>
 #include <linux/of_platform.h>
+#include <linux/slab.h>
 #include <linux/version.h>
 #include <media/v4l2-common.h>
 #include <media/v4l2-device.h>