Add missing method stub for _GerritCodereviewImpl.

git_cl.py

Index: git_cl.py
diff --git a/git_cl.py b/git_cl.py
index 45dace5f295929b90dd63205860a0047b9a10e0e..49d11c1a2aa5cf5a61c626f7621c6078d538dc19 100755
--- a/git_cl.py
+++ b/git_cl.py
@@ -2175,6 +2175,13 @@ class _GerritChangelistImpl(_ChangelistCodereviewBase):
   def CloseIssue(self):
     gerrit_util.AbandonChange(self._GetGerritHost(), self.GetIssue(), msg='')
 
+  def GetApprovingReviewers(self):
+    """Returns a list of reviewers approving the change.
+
+    Note: not necessarily committers.
+    """
+    raise NotImplementedError()
+
   def SubmitIssue(self, wait_for_merge=True):
     gerrit_util.SubmitChange(self._GetGerritHost(), self.GetIssue(),
                              wait_for_merge=wait_for_merge)