ASoC: omap-pcm: Include omap-pcm.h

omap_pcm_platform_register() is declared in omap-pcm.h and defined in
omap-pcm.c. To make sure that the function signature matches for both omap-pcm.c
should include omap-pcm.h

Fixes the following warning from sparse:
	sound/soc/omap/omap-pcm.c:235:5: warning: symbol
	'omap_pcm_platform_register' was not declared. Should it be static?

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@linaro.org>