ASoC: mediatek: mt8186: set variable aud_pinctrl to static

smatch reports
sound/soc/mediatek/mt8186/mt8186-afe-gpio.c:14:16: warning: symbol
  'aud_pinctrl' was not declared. Should it be static?

This variable is only used in one file so should be static.
Signed-off-by: Tom Rix <trix@redhat.com>
Link: https://lore.kernel.org/r/20230407115553.1968111-1-trix@redhat.comSigned-off-by: Mark Brown <broonie@kernel.org>

sound/soc/mediatek/mt8186/mt8186-afe-gpio.c

@@ -11,7 +11,7 @@
 #include "mt8186-afe-common.h"
 #include "mt8186-afe-gpio.h"
 
-struct pinctrl *aud_pinctrl;
+static struct pinctrl *aud_pinctrl;
 
 enum mt8186_afe_gpio {
 	MT8186_AFE_GPIO_CLK_MOSI_OFF,