Make wait_for_heatup volatile

Marlin/Marlin.h

 extern float sw_endstop_max[3]; // axis[n].sw_endstop_max
 extern bool axis_known_position[3]; // axis[n].is_known
 extern bool axis_homed[3]; // axis[n].is_homed
-extern bool wait_for_heatup;
+extern volatile bool wait_for_heatup;
 
 // GCode support for external objects
 bool code_seen(char);

Marlin/Marlin_main.cpp

 // Relative Mode. Enable with G91, disable with G90.
 static bool relative_mode = false;
 
-bool wait_for_heatup = true;
+volatile bool wait_for_heatup = true;
 
 const char errormagic[] PROGMEM = "Error:";
 const char echomagic[] PROGMEM = "echo:";

Marlin/ultralcd.h

   extern int absPreheatHPBTemp;
   extern int absPreheatFanSpeed;
 
-  extern bool wait_for_heatup;
-
   #if ENABLED(FILAMENT_LCD_DISPLAY)
     extern millis_t previous_lcd_status_ms;
   #endif