HID: 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>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

drivers/hid/hid-roccat-kone.c

@@ -35,6 +35,7 @@
 #include <linux/hid.h>
 #include <linux/usb.h>
 #include <linux/module.h>
+#include <linux/slab.h>
 #include "hid-ids.h"
 #include "hid-roccat-kone.h"