thermal: tsens: Fix exported function marked as static

This fixes the following warning:

../drivers/thermal/tsens-mtc.c:25:12: warning: unused function 'tsens_mtc_reset_history_counter' [-Wunused-function]
static int tsens_mtc_reset_history_counter(unsigned int zone)
           ^

Change-Id: I4dae50719d0d366ff999bc871e55300c8157ff8e