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]

libs/graph

@@ -1 +1 @@
-Subproject commit ad20c88c2005b8a612e5d3e64805eb77db1ac513
+Subproject commit f7fa2e2942983bbc4a748853ff74c5aec4a24e15