[ALSA] fix compiler warning

GUS Library
This patch fixes a compiler warning if sequencer is disabled.
Signed-off-by: Henrik Kretzschmar <henne@nachtwindheim.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>

sound/isa/gus/gus_main.c

@@ -417,11 +417,13 @@ static int snd_gus_check_version(snd_gus_card_t * gus)
 	return 0;
 }
 
+#if defined(CONFIG_SND_SEQUENCER) || (defined(MODULE) && defined(CONFIG_SND_SEQUENCER_MODULE))
 static void snd_gus_seq_dev_free(snd_seq_device_t *seq_dev)
 {
 	snd_gus_card_t *gus = seq_dev->private_data;
 	gus->seq_dev = NULL;
 }
+#endif
 
 int snd_gus_initialize(snd_gus_card_t *gus)
 {