Update gfxNullDevice.cpp

ubuntu gcc crash on this function

Engine/source/gfx/Null/gfxNullDevice.cpp

@@ -68,7 +68,7 @@ public:
    GFXNullTextureObject(GFXDevice * aDevice, GFXTextureProfile *profile); 
    ~GFXNullTextureObject() { kill(); };
 
-   void pureVirtualCrash() override { };
+   virtual void pureVirtualCrash() { };
 
    GFXLockedRect * lock( U32 mipLevel = 0, RectI *inRect = NULL ) override { return NULL; };
    void unlock( U32 mipLevel = 0) override {};