message: core: remove designated inits, fix sysid/compid
  Designated initializers don't compile with MSVC yet.
  And while I was at it I also realized that the target sysid/compid was
  used for target and origin which had to be wrong.
diff: @@ -257,18 +257,15 @@ void MAVLinkMissionTransfer::UploadWorkItem::process_mission_request(
  mavlink_mission_request_t request;
  mavlink_msg_mission_request_decode(&request_message, &request);
  - mavlink_mission_request_int_t request_int{
  - .seq = request.seq,
  - .target_system = request.target_system,
  - .target_component = request.target_component,
  - .mission_type = request.mission_type};
  -
  mavlink_message_t request_int_message;
  - mavlink_msg_mission_request_int_encode(
  - request_int.target_system,
  - request_int.target_component,
  + mavlink_msg_mission_request_int_pack(
  + request_message.sysid,
  + request_message.compid,
  &request_int_message,
  - &request_int);
  + request.target_system,
  + request.target_component,
  + request.seq,
  + request.mission_type);
  process_mission_request_int(request_int_message);
  } else {
  