Move G20_G21 to cpp

Marlin/src/Marlin.cpp

  ***************** GCode Handlers *****************
  **************************************************/
 
-#if ENABLED(INCH_MODE_SUPPORT)
-  #include "gcode/units/G20_G21.h"
-#endif
-
 #if ENABLED(NOZZLE_PARK_FEATURE)
   #include "gcode/feature/pause/G27.h"
 #endif

Marlin/src/gcode/gcode.cpp

 //
 // Placeholders for non-migrated codes
 //
-extern void gcode_G20();
-extern void gcode_G21();
 extern void gcode_G27();
 extern void gcode_G30();
 extern void gcode_G31();
 
       #if ENABLED(INCH_MODE_SUPPORT)
         case 20: // G20: Inch Mode
-          gcode_G20();
+          G20();
           break;
 
         case 21: // G21: MM Mode
-          gcode_G21();
+          G21();
           break;
       #endif // INCH_MODE_SUPPORT
 

Marlin/src/gcode/units/G20_G21.h → Marlin/src/gcode/units/G20_G21.cpp

  *
  */
 
+#include "../../inc/MarlinConfig.h"
+
+#if ENABLED(INCH_MODE_SUPPORT)
+
+#include "../gcode.h"
+
 /**
  * G20: Set input mode to inches
  */
-void gcode_G20() { parser.set_input_linear_units(LINEARUNIT_INCH); }
+void GcodeSuite::G20() { parser.set_input_linear_units(LINEARUNIT_INCH); }
 
 /**
  * G21: Set input mode to millimeters
  */
-void gcode_G21() { parser.set_input_linear_units(LINEARUNIT_MM); }
+void GcodeSuite::G21() { parser.set_input_linear_units(LINEARUNIT_MM); }
+
+#endif // INCH_MODE_SUPPORT