Bed Heater monitoring in Controller Fan

In some cases the Bed Heater FET heats up more then stepper drivers, so
this change add the bed monitoring to the controller fan. As soon as the
bed heater is turned on, the controller fan will run as well.

Marlin/temperature.h

@@ -45,6 +45,10 @@ extern float current_temperature_bed;
   extern float redundant_temperature;
 #endif
 
+#if defined(CONTROLLERFAN_PIN) && CONTROLLERFAN_PIN > -1
+  extern unsigned char soft_pwm_bed;
+#endif
+
 #ifdef PIDTEMP
   extern float Kp,Ki,Kd,Kc;
   float scalePID_i(float i);