Sphinx 1.7 removed sphinx.util.compat.Directive so people
who have upgraded cannot build the documentation.  Switch to
docutils.parsers.rst.Directive which has been available since
docutils 0.5 released in 2009.

Bugzilla: https://bugzilla.opensuse.org/show_bug.cgi?id=1083694


Co-developed-by: Takashi Iwai <tiwai@suse.de>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Cc: stable@vger.kernel.org
Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>