media: v4l: cadence: include linux/slab.h

I ran into a randconfig build error with the new driver:

drivers/media/platform/cadence/cdns-csi2tx.c: In function 'csi2tx_probe':
drivers/media/platform/cadence/cdns-csi2tx.c:477:11: error: implicit declaration of function 'kzalloc'; did you mean 'd_alloc'? [-Werror=implicit-function-declaration]

kzalloc() is declared in linux/slab.h, so let's include this to make it
build in all configurations.

Fixes: 84b477e6d4bc ("media: v4l: cadence: Add Cadence MIPI-CSI2 TX driver")

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>