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/mtdcore.c b/drivers/mtd/mtdcore.c
index 3ae06c8..a1b8b70 100644
--- a/drivers/mtd/mtdcore.c
+++ b/drivers/mtd/mtdcore.c
@@ -21,6 +21,7 @@
 #include <linux/proc_fs.h>
 #include <linux/idr.h>
 #include <linux/backing-dev.h>
+#include <linux/gfp.h>
 
 #include <linux/mtd/mtd.h>