fix wrong cast - instead function pointer variable pointer was used

(cherry picked from commit f4801313)

core/base/src/TVirtualGL.cxx

@@ -28,7 +28,7 @@ AddPseudoGlobals() {
   // User "gCling" as synonym for "libCore static initialization has happened".
    // This code here must not trigger it.
    TGlobalMappedFunction::Add(new TGlobalMappedFunction("gGLManager", "TVirtualGL*",
-                                 (TGlobalMappedFunction::GlobalFunc_t)&gGLManager));
+                                 (TGlobalMappedFunction::GlobalFunc_t) &TGLManager::Instance));
 }
 } gAddPseudoGlobals;
 }