staging: dgnc: driver.c: Need to include slab.h

With 'allyesconfig', we get following error without this.

drivers/staging/dgnc/dgnc_driver.c: In function ‘dgnc_cleanup_board’:
drivers/staging/dgnc/dgnc_driver.c:459:3: error: implicit declaration of function ‘kfree’ [-Werror=implicit-function-declaration]
   kfree(brd->msgbuf_head);
   ^
drivers/staging/dgnc/dgnc_driver.c: In function ‘dgnc_driver_kzmalloc’:
drivers/staging/dgnc/dgnc_driver.c:905:2: error: implicit declaration of function ‘kmalloc’ [-Werror=implicit-function-declaration]
  void *p = kmalloc(size, priority);
Signed-off-by: Tushar Behera <tushar.behera@linaro.org>
Cc: Lidza Louina <lidza.louina@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/dgnc/dgnc_driver.c

@@ -34,6 +34,7 @@
 #include <linux/version.h>
 #include <linux/module.h>
 #include <linux/pci.h>
+#include <linux/slab.h>
 
 #if LINUX_VERSION_CODE >= KERNEL_VERSION(2,6,39)
 #include <linux/sched.h>