Fix Windows build when MAVSDK with SUPERBUILD is external project

[julianoes]

Does that make any sense? It seems to work.

[julianoes]

I would assume that's unrelated:

 [12:37:29|Info ] MAVSDK version: be404ace (mavsdk_impl.cpp:20)
[12:37:29|Error] bind error: Address already in use (udp_connection.cpp:85)
/__w/MAVSDK/MAVSDK/src/integration_tests/mission_transfer_lossy.cpp:21: Failure
Expected equality of these values:
  mavsdk.add_udp_connection()
    Which is: Bind error
  ConnectionResult::Success
    Which is: Success