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: ef1d87e0 ("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>