staging: dgap: Add missing #include <linux/slab.h>

drivers/staging/dgap/dgap_driver.c: In function ‘dgap_cleanup_module’:
drivers/staging/dgap/dgap_driver.c:423: error: implicit declaration of function ‘kfree’
drivers/staging/dgap/dgap_driver.c: In function ‘dgap_driver_kzmalloc’:
drivers/staging/dgap/dgap_driver.c:940: error: implicit declaration of function ‘kmalloc’
drivers/staging/dgap/dgap_driver.c:940: warning: initialization makes pointer from integer without a cast

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/dgap/dgap_driver.c

@@ -36,6 +36,7 @@
 #include <linux/module.h>
 #include <linux/pci.h>
 #include <linux/delay.h>	/* For udelay */
+#include <linux/slab.h>
 #include <asm/uaccess.h>	/* For copy_from_user/copy_to_user */
 
 #if LINUX_VERSION_CODE >= KERNEL_VERSION(2,6,39)