ALSA: usb-audio: include format.h in format.c

Just in case a prototype changes, we'll be warned. This also fixes a
sparse warning.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
diff --git a/sound/usb/format.c b/sound/usb/format.c
index 5b792d2..95d5d7c 100644
--- a/sound/usb/format.c
+++ b/sound/usb/format.c
@@ -30,6 +30,7 @@
 #include "helper.h"
 #include "debug.h"
 #include "clock.h"
+#include "format.h"
 
 /*
  * parse the audio format type I descriptor