[ALSA] make sound/pci/hda/patch_sigmatel.c:stac92xx_dmic_labels[] static

This patch makes the needlessly global stac92xx_dmic_labels[] static.

Signed-off-by: Adrian Bunk <bunk@stusta.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>