ba3d: Vector3D: declare copy assignement opr

following deprecation warnings

GUI/ba3d/def.h

@@ -34,6 +34,7 @@ struct Vector3D {
     Vector3D(float, float, float);
 
     Vector3D(const Vector3D&) = default;
+    Vector3D& operator=(const Vector3D&) = default;
 
     Vector3D(QVector3D const&);
     operator QVector3D() const;