[media] as3645a: Fix compilation by including slab.h

The as3645a driver calls kzalloc(). Include slab.h.

Reported-by: Fredrik Lingvall <fredrik.lingvall@gmail.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>