[ALSA] add struct snd_pcm_substream forward declaration

fixes: include/sound/pcm.h:62: warning: 'struct snd_pcm_substream' declared
inside parameter list

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>

include/sound/pcm.h

@@ -56,6 +56,8 @@ struct snd_pcm_hardware {
 	size_t fifo_size;		/* fifo size in bytes */
 };
 
+struct snd_pcm_substream;
+
 struct snd_pcm_ops {
 	int (*open)(struct snd_pcm_substream *substream);
 	int (*close)(struct snd_pcm_substream *substream);