Move G4 to cpp

Marlin/src/Marlin.cpp

  ***************** GCode Handlers *****************
  **************************************************/
 
-#include "gcode/motion/G4.h"
-
 #if ENABLED(BEZIER_CURVE_SUPPORT)
   #include "gcode/motion/G5.h"
 #endif

Marlin/src/gcode/gcode.cpp

 //
 // Placeholders for non-migrated codes
 //
-extern void gcode_G4();
 extern void gcode_G5();
 extern void gcode_G12();
 extern void gcode_G17();
 
       // G4 Dwell
       case 4:
-        gcode_G4();
+        G4();
         break;
 
       #if ENABLED(BEZIER_CURVE_SUPPORT)

Marlin/src/gcode/gcode.h

     #define KEEPALIVE_STATE(n) NOOP
   #endif
 
-  void dwell(millis_t time);
+  static void dwell(millis_t time);
 
 private:
 

Marlin/src/gcode/motion/G4.h → Marlin/src/gcode/motion/G4.cpp

  *
  */
 
+#include "../gcode.h"
+#include "../../module/stepper.h"
+#include "../../lcd/ultralcd.h"
+
 /**
  * G4: Dwell S<seconds> or P<milliseconds>
  */
-void gcode_G4() {
+void GcodeSuite::G4() {
   millis_t dwell_ms = 0;
 
   if (parser.seenval('P')) dwell_ms = parser.value_millis(); // milliseconds to wait
 
   if (!lcd_hasstatus()) LCD_MESSAGEPGM(MSG_DWELL);
 
-  gcode.dwell(dwell_ms);
+  dwell(dwell_ms);
 }