arm: update gfp/slab.h includes

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: NTejun Heo <tj@kernel.org>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Russell King <linux@arm.linux.org.uk>
Signed-off-by: NStephen Rothwell <sfr@canb.auug.org.au>

arch/arm/mach-vexpress/ct-ca9x4.c

@@ -2,6 +2,7 @@
  * Versatile Express Core Tile Cortex A9x4 Support
  */
 #include <linux/init.h>
+#include <linux/gfp.h>
 #include <linux/device.h>
 #include <linux/dma-mapping.h>
 #include <linux/platform_device.h>