Move M31 to cpp

Marlin/src/Marlin.cpp

  ***************** GCode Handlers *****************
  **************************************************/
 
-#include "gcode/stats/M31.h"              // M31: Get the time since the start of SD Print (or last M109)
-
 /**
  * Sensitive pin test for M42, M226
  */

Marlin/src/gcode/gcode.cpp

 // Placeholders for non-migrated codes
 //
 extern void gcode_M18_M84();
-extern void gcode_M31();
 extern void gcode_M42();
 extern void gcode_M43();
 extern void gcode_M48();
         case 928: M928(); break;  // M928: Start SD write
       #endif // SDSUPPORT
 
-      case 31: // M31: Report time since the start of SD print or last M109
-        gcode_M31(); break;
+      case 31: M31(); break;      // M31: Report time since the start of SD print or last M109
 
       case 42: // M42: Change pin state
         gcode_M42(); break;

Marlin/src/gcode/stats/M31.h → Marlin/src/gcode/stats/M31.cpp

  *
  */
 
+#include "../gcode.h"
+#include "../../core/serial.h"
+#include "../../module/printcounter.h"
+#include "../../libs/duration_t.h"
+#include "../../lcd/ultralcd.h"
+
 /**
  * M31: Get the time since the start of SD Print (or last M109)
  */
-void gcode_M31() {
+void GcodeSuite::M31() {
   char buffer[21];
   duration_t elapsed = print_job_timer.duration();
   elapsed.toString(buffer);