thermal: core: make thermal_emergency_poweroff static

Making thermal_emergency_poweroff static fixes sparse warning:

  drivers/thermal/thermal_core.c:6: warning: symbol
  'thermal_emergency_poweroff' was not declared. Should it be static?

Fixes: ef1d87e06ab4 ("thermal: core: Add a back up thermal shutdown mechanism")
Acked-by: Keerthy <j-keerthy@ti.com>
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>

drivers/thermal/thermal_core.c

@@ -359,7 +359,7 @@ static DECLARE_DELAYED_WORK(thermal_emergency_poweroff_work,
  * This may be called from any critical situation to trigger a system shutdown
  * after a known period of time. By default this is not scheduled.
  */
-void thermal_emergency_poweroff(void)
+static void thermal_emergency_poweroff(void)
 {
 	int poweroff_delay_ms = CONFIG_THERMAL_EMERGENCY_POWEROFF_DELAY_MS;
 	/*