ALSA: hda/realtek - Fix unused variable warning w/o CONFIG_LEDS_TRIGGER_AUDIO

Cover with a proper ifdef around the variable declaration for fixing
the following compilation warning without CONFIG_LEDS_TRIGGER_AUDIO:
  sound/pci/hda/patch_realtek.c: In function 'alc_fixup_hp_gpio_led':
  sound/pci/hda/patch_realtek.c:4134:6: warning: unused variable 'err' [-Wunused-variable]

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Fixes: 87dc36482c ("ALSA: hda/realtek - Add LED class support for micmute LED")
Link: https://lore.kernel.org/r/20200501072857.13720-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/pci/hda/patch_realtek.c

@@ -4131,7 +4131,9 @@ static void alc_fixup_hp_gpio_led(struct hda_codec *codec,
 				  unsigned int micmute_mask)
 {
 	struct alc_spec *spec = codec->spec;
+#if IS_REACHABLE(CONFIG_LEDS_TRIGGER_AUDIO)
 	int err;
+#endif
 
 	alc_fixup_gpio(codec, action, mute_mask | micmute_mask);