Properly declare namespace package

The 'sphinxcontrib' is declared a namespace package by setuptools means
via pbr.  Setuptools 0.7 or later mandates explicit declaration via
pkg_resources.declare_namespace (PEP-382).

Otherwise refuses to "setup.py build" on systems with setuptools>0.7.

Change-Id: If0d214a3b2d148450faa2b09292aafcdcf6837e9