ASoC: dapm: mark 'snd_soc_dapm_free_kcontrol' as static

The newly introduced function is declared as globally visible,
but is not declared in a header, causing a warning 'make W=1'
or 'make C=1':

sound/soc/soc-dapm.c:3782:1: warning: symbol 'snd_soc_dapm_free_kcontrol' was not declared. Should it be static?

The suggestion to make it static seems appropriate here, so let's
do that.

Fixes: 19ad683a ("ASoC: dapm: Avoid creating kcontrol for params")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mark Brown <broonie@kernel.org>