mingw: fix compile error in GraphicsWindowWin32.cpp
mingw: fix compile error in GraphicsWindowWin32.cpp

compilation error was introduced by msvc specific change
see https://github.com/openscenegraph/OpenSceneGraph/commit/bb169979a1054e2fa63a7ea82c9c8bec87e90494

scrawl/osg,scrawl/osg,openscenegraph/osg,openscenegraph/osg,scrawl/osg,openscenegraph/osg,scrawl/osg,openscenegraph/osg