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

Add struct snd_pcm_substream forward declaration

Fixes: 078a85f2 ("ASoC: dapm: Only power up active channels from a DAI")
Signed-off-by: Lucas Tanure <lucas.tanure@collabora.com>
Reviewed-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20230215132851.1626881-1-lucas.tanure@collabora.com


Signed-off-by: Mark Brown <broonie@kernel.org>