Add slab.h to fix ak881x build:

drivers/media/video/ak881x.c:265:error: implicit declaration of function 'kzalloc'
drivers/media/video/ak881x.c:265:warning: assignment makes pointer from integer without a cast
drivers/media/video/ak881x.c:283:error: implicit declaration of function 'kfree'
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>