ALSA: pcm: Build pcm notifier code conditionally

The PCM notifier code is used only by OSS emulation layer, so we can
build it conditionally for reducing the size.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
diff --git a/include/sound/pcm.h b/include/sound/pcm.h
index 361749e..eb16912 100644
--- a/include/sound/pcm.h
+++ b/include/sound/pcm.h
@@ -531,13 +531,6 @@ struct snd_pcm {
 #endif
 };
 
-struct snd_pcm_notify {
-	int (*n_register) (struct snd_pcm * pcm);
-	int (*n_disconnect) (struct snd_pcm * pcm);
-	int (*n_unregister) (struct snd_pcm * pcm);
-	struct list_head list;
-};
-
 /*
  *  Registering
  */
@@ -552,7 +545,15 @@ int snd_pcm_new_internal(struct snd_card *card, const char *id, int device,
 		struct snd_pcm **rpcm);
 int snd_pcm_new_stream(struct snd_pcm *pcm, int stream, int substream_count);
 
+#if IS_ENABLED(CONFIG_SND_PCM_OSS)
+struct snd_pcm_notify {
+	int (*n_register) (struct snd_pcm * pcm);
+	int (*n_disconnect) (struct snd_pcm * pcm);
+	int (*n_unregister) (struct snd_pcm * pcm);
+	struct list_head list;
+};
 int snd_pcm_notify(struct snd_pcm_notify *notify, int nfree);
+#endif
 
 /*
  *  Native I/O