[ALSA] Fix Oops at rmmod with CONFIG_SND_VERBOSE_PROCFS=n

Fixed Oops at rmmod with CONFIG_SND_VERBOSE_PROCFS=n.
Add ifdef to struct fields for optimization and better compile
checks.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
diff --git a/include/sound/pcm_oss.h b/include/sound/pcm_oss.h
index 39df2ba..c854647 100644
--- a/include/sound/pcm_oss.h
+++ b/include/sound/pcm_oss.h
@@ -75,7 +75,9 @@
 struct snd_pcm_oss_stream {
 	struct snd_pcm_oss_setup *setup_list;	/* setup list */
 	struct mutex setup_mutex;
+#ifdef CONFIG_SND_VERBOSE_PROCFS
 	struct snd_info_entry *proc_entry;
+#endif
 };
 
 struct snd_pcm_oss {