Added missing self argument

Obviously the tests don't call this. This is probably because they use
DB.storage (rather than DB._mvcc_storage).

src/ZODB/mvccadapter.py

@@ -34,7 +34,7 @@ class Base(object):
 
         raise AttributeError(name)
 
-    def __len__():
+    def __len__(self):
         return len(self._storage)
 
 class MVCCAdapter(Base):