ALSA: usb - use of kmalloc/kfree requires the include of slab.h

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
diff --git a/sound/usb/helper.c b/sound/usb/helper.c
index ba7dba4..d48d6f8 100644
--- a/sound/usb/helper.c
+++ b/sound/usb/helper.c
@@ -16,6 +16,7 @@
  */
 
 #include <linux/init.h>
+#include <linux/slab.h>
 #include <linux/usb.h>
 
 #include "usbaudio.h"