ASoC: soc-dapm.h: fixup warning struct snd_pcm_substream not declared

Add struct snd_pcm_substream forward declaration

Fixes: 078a85f2806f ("ASoC: dapm: Only power up active channels from a DAI")
Signed-off-by: Lucas Tanure <[e-mail]>
Reviewed-by: Charles Keepax <[e-mail]>
Reviewed-by: AngeloGioacchino Del Regno <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Mark Brown <[e-mail]>

include/sound/soc-dapm.h

···
 #include <sound/asoc.h>
 
 struct device;
+struct snd_pcm_substream;
 struct snd_soc_pcm_runtime;
 struct soc_enum;