Works around a compiler warning with OSG 3.4:
warning: base class 'class osg::Callback' should be explicitly initialized in the copy constructor [-Wextra]
With no default argument for osg::CopyOp&, the compiler no longer sees the function as a real copy constructor and stops warning about the missing virtual initializations.
We don't care about this warning because there is nothing interesting to initialize in the osg::NodeCallback base anyway.
A proper fix for the warning would require to inserting OSG_VERSION conditional compiling all over the place, that is as long as we are still supporting OSG 3.2.
