Only display poweron/poweroff if PS_ON_PIN > -1

Marlin/ultralcd.cpp

     MENU_ITEM(function, MSG_PREHEAT_PLA, lcd_preheat_pla);
     MENU_ITEM(function, MSG_PREHEAT_ABS, lcd_preheat_abs);
     MENU_ITEM(function, MSG_COOLDOWN, lcd_cooldown);
+#if PS_ON_PIN > -1
     if (powersupply)
     {
         MENU_ITEM(gcode, MSG_SWITCH_PS_OFF, PSTR("M81"));
     }else{
         MENU_ITEM(gcode, MSG_SWITCH_PS_ON, PSTR("M80"));
     }
+#endif
     MENU_ITEM(submenu, MSG_MOVE_AXIS, lcd_move_menu);
     END_MENU();
 }