ASoC: trace: Make sure trace header doesnt depend on any headers

Fix build so that asoc trace event header doesn't depend on other headers.
Signed-off-by: Liam Girdwood <liam.r.girdwood@linux.intel.com>
Signed-off-by: Mark Brown <broonie@linaro.org>

include/trace/events/asoc.h

@@ -14,6 +14,7 @@ struct snd_soc_codec;
 struct snd_soc_platform;
 struct snd_soc_card;
 struct snd_soc_dapm_widget;
+struct snd_soc_dapm_path;
 
 /*
  * Log register events