libstdc++: Define _versioned_namespace in xmethods.py

flake8 pointed out that is_specialization_of in xmethods.py looks at a
global that wasn't added to the file.  This patch correct the
oversight.

libstdc++-v3/ChangeLog:

	* python/libstdcxx/v6/xmethods.py (_versioned_namespace):
	Define.