[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>