Move M999 to cpp

Marlin/src/Marlin.cpp

   SYNC_PLAN_POSITION_KINEMATIC();
 }
 
-#include "gcode/control/M999.h"
-
 #include "gcode/control/T.h"
 
 #if ENABLED(USE_CONTROLLER_FAN)

Marlin/src/gcode/control/M999.h → Marlin/src/gcode/control/M999.cpp

  *
  */
 
-#include "../queue.h"
+#include "../gcode.h"
+
+#include "../../lcd/ultralcd.h" // for lcd_reset_alert_level
+#include "../../Marlin.h"       // for Running
+#include "../queue.h"           // for flush_and_request_resend
 
 /**
  * M999: Restart after being stopped
  * existing command buffer.
  *
  */
-void gcode_M999() {
+void GcodeSuite::M999() {
   Running = true;
   lcd_reset_alert_level();
 

Marlin/src/gcode/gcode.cpp

 //
 // Placeholders for non-migrated codes
 //
-extern void gcode_M999();
 extern void gcode_T(uint8_t tmp_extruder);
 
 #if ENABLED(M100_FREE_MEMORY_WATCHER)
         case 869: M869(); break; // M869: Report axis error
       #endif
 
-      case 999: // M999: Restart after being Stopped
-        gcode_M999();
-        break;
+      case 999: M999(); break;  // M999: Restart after being Stopped
     }
     break;