Fix header to hopefully permit iOS builds to work again.
GUIShader.cpp added #include windowing/egl/WinSystemEGL.h inside a
but also need the header windowing/osx/WinSystemIOS.h instead. The only thing
GUIShader.cpp needed was g_windowing.GetViewPort, which is provided by the
common base class CRenderSystemGLES of g_windowing in both cases, so I think
it should be sufficient to use windowing/WindowingFactory.h instead, which is
abstracted away from the other header files.