ASoC: dapm: Staticise dapm_mark_dirty()

The function is not called outside soc-dapm.c so there is no need for it
to be exported.

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

diff --cc include/sound/soc-dapm.h
Simple merge

diff --cc sound/soc/soc-dapm.c
Simple merge