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);
     ^

---
 drivers/media/usb/cx231xx/cx231xx-417.c | 1 +
 1 file changed, 1 insertion(+)

index b80e6857e2eb..2f3b0564d676 100644
@@ -29,6 +29,7 @@
 #include <linux/delay.h>
 #include <linux/device.h>
 #include <linux/firmware.h>
+#include <linux/slab.h>
 #include <linux/vmalloc.h>
 #include <media/v4l2-common.h>
 #include <media/v4l2-ioctl.h>

2.17.0