mtd: 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: Tejun Heo <tj@kernel.org>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
diff --git a/drivers/mtd/nand/r852.c b/drivers/mtd/nand/r852.c
index 6dfbb47..78a4232 100644
--- a/drivers/mtd/nand/r852.c
+++ b/drivers/mtd/nand/r852.c
@@ -15,6 +15,7 @@
 #include <linux/pci.h>
 #include <linux/pci_ids.h>
 #include <linux/delay.h>
+#include <linux/slab.h>
 #include <asm/byteorder.h>
 #include <linux/sched.h>
 #include "sm_common.h"