Give the node _Default class a destBranchExpr

This is to avoid the following AttributeError:

line 681, in _ParseProject
AttributeError: '_Default' object has no attribute 'destBranchExpr'

Change-Id: Ia9f7e2cce1409d22d71bc8a74b33edf2b27702ca

manifest_xml.py

@@ -45,6 +45,7 @@ class _Default(object):
   """Project defaults within the manifest."""
 
   revisionExpr = None
+  destBranchExpr = None
   remote = None
   sync_j = 1
   sync_c = False