media: usb: cx231xx-417: include linux/slab.h header

cx231-417 uses kmalloc/kfree functions, so slab header needs to be
included in order to fix the following build errors:
drivers/media/usb/cx231xx/cx231xx-417.c: In function ‘cx231xx_bulk_copy’:
  CC      drivers/media/platform/qcom/venus/firmware.o
drivers/media/usb/cx231xx/cx231xx-417.c:1389:11: error: implicit declaration of function ‘kmalloc’; did you mean ‘vmalloc’? [-Werror=implicit-function-declaration]
  buffer = kmalloc(buffer_size, GFP_ATOMIC);
           ^~~~~~~
           vmalloc
drivers/media/usb/cx231xx/cx231xx-417.c:1389:9: warning: assignment makes pointer from integer without a cast [-Wint-conversion]
  buffer = kmalloc(buffer_size, GFP_ATOMIC);
         ^
drivers/media/usb/cx231xx/cx231xx-417.c:1400:2: error: implicit declaration of function ‘kfree’; did you mean ‘vfree’? [-Werror=implicit-function-declaration]
  kfree(buffer);
  ^~~~~
  vfree
drivers/media/usb/cx231xx/cx231xx-417.c: In function ‘mpeg_open’:
drivers/media/usb/cx231xx/cx231xx-417.c:1713:7: error: implicit declaration of function ‘kzalloc’; did you mean ‘vzalloc’? [-Werror=implicit-function-declaration]
  fh = kzalloc(sizeof(*fh), GFP_KERNEL);
       ^~~~~~~
       vzalloc
drivers/media/usb/cx231xx/cx231xx-417.c:1713:5: warning: assignment makes pointer from integer without a cast [-Wint-conversion]
  fh = kzalloc(sizeof(*fh), GFP_KERNEL);
     ^

Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Signed-off-by: Hans Verkuil <hansverk@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>