Unifying access to GLEW from the Blender Game Engine

Note: This is not about functionality, but about using the same stub file
we are using in Blender for the game engine in blender2.8.

source/gameengine/Rasterizer/RAS_OpenGLRasterizer/RAS_OpenGLOffScreen.cpp

@@ -25,7 +25,7 @@
  * ***** END GPL LICENSE BLOCK *****
  */
 
-#include "glew-mx.h"
+#include "GPU_glew.h"
 
 #include <stdio.h>