ALSA: jack: Add dummy snd_jack_set_key() definition

For fixing a build error with CONFIG_SND_JACK=n
  sound/soc/codecs/ts3a227e.c:223:2: error: implicit declaration of function ‘snd_jack_set_key’ [-Werror=implicit-function-declaration]

Signed-off-by: Takashi Iwai <tiwai@suse.de>