Enable WATCH_TEMP_PERIOD by default

Marlin/temperature.h

@@ -130,7 +130,6 @@ HOTEND_ROUTINES(0);
 
 int getHeaterPower(int heater);
 void disable_all_heaters();
-void setWatch();
 void updatePID();
 
 void PID_autotune(float temp, int extruder, int ncycles);
@@ -138,6 +137,10 @@ void PID_autotune(float temp, int extruder, int ncycles);
 void setExtruderAutoFanState(int pin, bool state);
 void checkExtruderAutoFans();
 
+#ifdef WATCH_TEMP_PERIOD
+  void start_watching_heaters();
+#endif
+
 FORCE_INLINE void autotempShutdown() {
   #ifdef AUTOTEMP
     if (autotemp_enabled) {