gpio: Update gfp/slab.h includes for rdc321x

Implicit slab.h inclusion via percpu.h is about to go away.  Make sure
gfp.h or slab.h is included as necessary.
Signed-off-by: NSamuel Ortiz <sameo@linux.intel.com>
Signed-off-by: NTejun Heo <tj@kernel.org>

drivers/gpio/rdc321x-gpio.c

@@ -27,6 +27,7 @@
 #include <linux/pci.h>
 #include <linux/gpio.h>
 #include <linux/mfd/rdc321x.h>
+#include <linux/slab.h>
 
 struct rdc321x_gpio {
 	spinlock_t		lock;