[media] media: pms.c needs linux/slab.h

drivers/media/video/pms.c uses kzalloc() and kfree() so it should
include <linux/slab.h> to fix build errors and a warning.

drivers/media/video/pms.c:1047:2: error: implicit declaration of function 'kzalloc'
drivers/media/video/pms.c:1047:6: warning: assignment makes pointer from integer without a cast
drivers/media/video/pms.c:1116:2: error: implicit declaration of function 'kfree'

Found in mmotm but applies to mainline.

Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>