Fix extrudernum in MSG_INVALID_EXTRUDER

SERIAL_ECHO is not very good in printing uint8_t - so cast. to int.

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;
     }
   }