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>
Cc: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>