ALSA: usb-audio: export snd_usb_feature_unit_ctl

In order to allow quirks functions to hook up to the standard feature
unit op tables, this patch exports a pointer to the struct that is used
internally.

That way, all the code handling the control can be kept private, and
external code can reference the symbol to re-use it.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
diff --git a/sound/usb/card.c b/sound/usb/card.c
index 84a5ce7..220c616 100644
--- a/sound/usb/card.c
+++ b/sound/usb/card.c
@@ -48,6 +48,7 @@
 #include <linux/usb/audio.h>
 #include <linux/usb/audio-v2.h>
 
+#include <sound/control.h>
 #include <sound/core.h>
 #include <sound/info.h>
 #include <sound/pcm.h>