ALSA: usb - 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: Takashi Iwai <tiwai@suse.de>
diff --git a/sound/usb/urb.c b/sound/usb/urb.c
index e2b7c478..5570a2b 100644
--- a/sound/usb/urb.c
+++ b/sound/usb/urb.c
@@ -15,6 +15,7 @@
  *
  */
 
+#include <linux/gfp.h>
 #include <linux/init.h>
 #include <linux/usb.h>
 #include <linux/usb/audio.h>