ASoC: jack: Fix multiple definition of `snd_soc_jack_add_gpiods'

Commit f025d3b9

 ("ASoC: jack: Add support for GPIO descriptor defined
jack pins") caused build error when CONFIG_GPIOLIB is not set:

sound/include/sound/soc.h:470: multiple definition of `snd_soc_jack_add_gpiods'
sound/soc/soc-core.o:sound/include/sound/soc.h:470: first defined here
make[2]: *** [sound/soc/snd-soc-core.o] Error 1
make[2]: *** Waiting for unfinished jobs....
make[1]: *** [sound/soc] Error 2
make: *** [sound] Error 2

Fix this by marking snd_soc_jack_add_gpiods() as static inline in soc.h.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Signed-off-by: Mark Brown <broonie@linaro.org>