Normally you would parent the crosshair to the camera, not the other way round. You may however parent the crosshair to the ship so it will move with it, and then correct the rotation each frame using the rotation angles of the camera, eg:
rotateentity cross,entitypitch(cam),entityyaw(cam),entityroll(cam),1
Tho, since the problem is solved, it doesn't matter that much.
rotateentity cross,entitypitch(cam),entityyaw(cam),entityroll(cam),1
Tho, since the problem is solved, it doesn't matter that much.