changed to mutable iterator, to match return type of std::set::find, just

in case the std implementation does not provide constant to mutable
iterator conversions for std::set.


[SVN r22081]

include/boost/graph/detail/adjacency_list.hpp

@@ -1321,7 +1321,7 @@ namespace boost {
                     boost::disallow_parallel_edge_tag) const
       {
         bool found;
-        typename Config::OutEdgeList::const_iterator 
+        typename Config::OutEdgeList::iterator 
           i = g.out_edge_list(u).find(StoredEdge(v)),
           end = g.out_edge_list(u).end();
         found = (i != end);