ALSA: usb-audio: Declare the common variable in header file

Declare snd_usb_feature_unit_ctl properly in mixer.h.  Otherwise it's
error-prone.

This fixes the sparse warning:
  sound/usb/mixer.c:1464:25: warning: symbol 'snd_usb_feature_unit_ctl' was not declared. Should it be static?
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/mixer.h

@@ -109,4 +109,6 @@ int snd_usb_get_cur_mix_value(struct usb_mixer_elem_info *cval,
 
 extern void snd_usb_mixer_elem_free(struct snd_kcontrol *kctl);
 
+extern struct snd_kcontrol_new *snd_usb_feature_unit_ctl;
+
 #endif /* __USBMIXER_H */

sound/usb/mixer_quirks.c

@@ -47,8 +47,6 @@
 #include "mixer_us16x08.h"
 #include "helper.h"
 
-extern struct snd_kcontrol_new *snd_usb_feature_unit_ctl;
-
 struct std_mono_table {
 	unsigned int unitid, control, cmask;
 	int val_type;