Merge pull request #3187 from AnHardt/target_extruder_number

Fix extrudernum in MSG_INVALID_EXTRUDER

Marlin/Marlin_main.cpp

       SERIAL_CHAR('M');
       SERIAL_ECHO(code);
       SERIAL_ECHOPGM(" " MSG_INVALID_EXTRUDER " ");
-      SERIAL_ECHOLN(target_extruder);
+      SERIAL_ECHOLN((int)target_extruder);
       return true;
     }
   }