Move M304 to cpp

Marlin/src/Marlin.cpp

   return false;
 }
 
-#if ENABLED(PIDTEMPBED)
-  #include "gcode/config/M304.h"
-#endif
-
 #if defined(CHDK) || HAS_PHOTOGRAPH
   #include "gcode/feature/camera/M240.h"
 #endif

Marlin/src/gcode/config/M304.h → Marlin/src/gcode/config/M304.cpp

  *
  */
 
-void gcode_M304() {
+#include "../../inc/MarlinConfig.h"
+
+#if ENABLED(PIDTEMPBED)
+
+#include "../gcode.h"
+#include "../../module/temperature.h"
+
+void GcodeSuite::M304() {
   if (parser.seen('P')) thermalManager.bedKp = parser.value_float();
   if (parser.seen('I')) thermalManager.bedKi = scalePID_i(parser.value_float());
   if (parser.seen('D')) thermalManager.bedKd = scalePID_d(parser.value_float());
   SERIAL_ECHOPAIR(" i:", unscalePID_i(thermalManager.bedKi));
   SERIAL_ECHOLNPAIR(" d:", unscalePID_d(thermalManager.bedKd));
 }
+
+#endif // PIDTEMPBED

Marlin/src/gcode/gcode.cpp

 extern void gcode_M240();
 extern void gcode_M250();
 extern void gcode_M302();
-extern void gcode_M304();
 extern void gcode_M350();
 extern void gcode_M351();
 extern void gcode_M355();
       #endif
 
       #if ENABLED(PIDTEMPBED)
-        case 304: // M304: Set bed PID parameters
-          gcode_M304();
-          break;
-      #endif // PIDTEMPBED
+        case 304: M304(); break;  // M304: Set bed PID parameters
+      #endif
 
       #if defined(CHDK) || HAS_PHOTOGRAPH
         case 240: // M240: Trigger a camera by emulating a Canon RC-1 : http://www.doc-diy.net/photo/rc-1_hacked/