ALSA: usb pcm: use of kmalloc requires the include of slab.h

Signed-off-by: NStephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: NTakashi Iwai <tiwai@suse.de>

sound/usb/pcm.c

@@ -15,6 +15,7 @@
  */
 
 #include <linux/init.h>
+#include <linux/slab.h>
 #include <linux/usb.h>
 #include <linux/usb/audio.h>
 #include <linux/usb/audio-v2.h>