Missing oxygene.mod message

Happened the whole time, in any campaign, in any level 😔️.
![Schermata da 2023-06-05 13-47-54](https://github.com/cxong/cdogs-sdl/assets/123581658/460b5c54-eef0-41fc-93e2-ef7ae2864e01)
![Schermata da 2023-06-05 14-01-38](https://github.com/cxong/cdogs-sdl/assets/123581658/65b28c52-f551-48c8-9f9e-456667d3ab13)