Apply const to manage_inactivity

Marlin/src/Marlin.cpp

  *  - Check if cooling fan needs to be switched on
  *  - Check if an idle but hot extruder needs filament extruded (EXTRUDER_RUNOUT_PREVENT)
  */
-void manage_inactivity(bool ignore_stepper_queue/*=false*/) {
+void manage_inactivity(const bool ignore_stepper_queue/*=false*/) {
 
   #if ENABLED(FILAMENT_RUNOUT_SENSOR)
     runout.run();

Marlin/src/Marlin.h

   #endif
 );
 
-void manage_inactivity(bool ignore_stepper_queue = false);
+void manage_inactivity(const bool ignore_stepper_queue=false);
 
 #if HAS_X2_ENABLE
   #define  enable_X() do{ X_ENABLE_WRITE( X_ENABLE_ON); X2_ENABLE_WRITE( X_ENABLE_ON); }while(0)