Fix GraphTraits for "const CallGraphNode *" and "const CallGraph *"

The specializations were broken. For example,

void foo(const CallGraph *G) {
  auto I = GraphTraits<const CallGraph *>::nodes_begin(G);
  auto K = I++;

  ...
}

or

void bar(const CallGraphNode *N) {
  auto I = GraphTraits<const CallGraphNode *>::nodes_begin(G);
  auto K = I++;

  ....
}

would not compile.

Patch by Speziale Ettore!