message: mavlink_passthrough: fix forward declaration
diff: namespace mavsdk {
  -template class CallbackList<void(const mavlink_message_t&)>;
  +template class CallbackList<const mavlink_message_t&>;
  MavlinkPassthroughImpl::MavlinkPassthroughImpl(System& system) : PluginImplBase(system)
  {
  