Fix compiler warning when fullscreen demo is built w/o OpenGL

Warning: -Wsuggest-override

test/fullscreen.cxx

@@ -99,7 +99,7 @@ void shape_window::draw() {
 #include <FL/fl_draw.H>
 
 class shape_window : public Fl_Window {
-  void draw();
+  void draw() FL_OVERRIDE;
 public:
   int sides;
   shape_window(int x,int y,int w,int h,const char *l=0);