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>