ALSA: xen: remove 'set but not used' warning

Fix W=1 warning. Mark the 'req' variable as __always_unused.

sound/xen/xen_snd_front.c: In function ‘xen_snd_front_stream_close’:
sound/xen/xen_snd_front.c:117:21: warning: variable ‘req’ set but not
used [-Wunused-but-set-variable]
  117 |  struct xensnd_req *req;
      |                     ^~~

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20200702193604.169059-18-pierre-louis.bossart@linux.intel.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>