error: replace GIT_REVWALKOVER with GIT_ITEROVER.

This catches up with changes in libgit2:

  commit f335ecd6e126aa9dea28786522c0e6ce71596e91
  Author: Russell Belfer <rb@github.com>
  Date:   Thu Aug 30 14:24:16 2012 -0700

src/pygit2/error.c

@@ -55,7 +55,7 @@ PyObject * Error_type(int type)
             return GitError;
 
         /** No entries left in ref walker */
-        case GIT_REVWALKOVER:
+        case GIT_ITEROVER:
             return PyExc_StopIteration;
     }