Incorrect CmdMsg::DATA_LENGTH used for HornCmdMsg when it is pushed into rx_list

[jilinzhouas]

The issue is self-explained in the code (in pacmod3_node.cpp):

    rx_list.emplace(
      HornCmdMsg::CAN_ID,
      std::shared_ptr<LockedData>(new LockedData(HeadlightCmdMsg::DATA_LENGTH)));

Although HeadlightCmdMsg::DATA_LENGTH is equal to HornCmdMsg::DATA_LENGTH, it should be changed.