Fix clang warning by avoiding GNU extension.

../OpenFlipper/libs_required/ACG/GL/globjects.hh:154:25: warning: arithmetic on a null pointer treated as a cast from integer to pointer is a GNU extension [-Wnull-pointer-arithmetic]
    return ((char*)NULL + _offset);

libs_required/ACG/GL/globjects.hh

@@ -151,7 +151,7 @@ public:
 
   char* offset(unsigned int _offset) const
   {
-    return ((char*)NULL + _offset);
+    return reinterpret_cast<char*>(_offset);
   }
 
   GLuint id() const {return vbo;}