ASoC: dapm: declare missing structure prototypes

To fix compilation warnings:

- struct 'snd_soc_pcm_runtime' declared inside parameter list will not
  be visible outside of this definition or declaration
- struct 'soc_enum' declared inside parameter list will not be visible
  outside of this definition or declaration

Declares the missing structure prototypes.
Signed-off-by: Tzung-Bi Shih <tzungbi@google.com>
Link: https://lore.kernel.org/r/20200625153543.85039-3-tzungbi@google.comSigned-off-by: Mark Brown <broonie@kernel.org>

include/sound/soc-dapm.h

@@ -16,6 +16,8 @@
 #include <sound/asoc.h>
 
 struct device;
+struct snd_soc_pcm_runtime;
+struct soc_enum;
 
 /* widget has no PM register bit */
 #define SND_SOC_NOPM	-1