Include missing header

I thought I'd seen this class defined in one of the existing headers
with a different name, but I was muddling its forward declaration and a
different class being in a non-obvious header.

components/sceneutil/glextensions.cpp

@@ -1,5 +1,7 @@
 #include "glextensions.hpp"
 
+#include <osg/GraphicsContext>
+
 namespace SceneUtil
 {
     namespace