Depend on docbook-dtds for local validation
    
    Added Requires: docbook-dtds to the gnome-doc-utils-stylesheets subpackage
    so that xsltproc validation wouldn't have to download DTDs.
    
    Koji has no network access so this was resulting in xsltproc warnings:
    > warning: failed to load external entity "http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd"