V4L/DVB: vivi and mem2mem_testdev need slab.h to build

Fix vivi and mem2mem_testdev build errors: need to #include <linux/slab.h>:

drivers/media/video/vivi.c:1144: error: implicit declaration of function 'kfree'
drivers/media/video/vivi.c:1156: error: implicit declaration of function 'kzalloc'
drivers/media/video/vivi.c:1156: warning: assignment makes pointer from integer without a cast
drivers/media/video/mem2mem_testdev.c:862: error: implicit declaration of function 'kzalloc'
drivers/media/video/mem2mem_testdev.c:862: warning: assignment makes pointer from integer without a cast
drivers/media/video/mem2mem_testdev.c:874: error: implicit declaration of function 'kfree'
drivers/media/video/mem2mem_testdev.c:944: warning: assignment makes pointer from integer without a cast

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>