main/docbook-xsl: make docbook-xsl depend on both -ns and -nons
It seems that many packages still use the old URI http://docbook.sourceforge.net/release/xsl and for some reason don't mind that we previously pointed it to a different variant of the XSL templates (or fail silently as PostgreSQL used to?). Since this URI is now registered for docbook-xsl-nons and docbook-xsl installs only docbook-xsl-ns, it broke the build of many packages.
They can be installed at the same time, so there's no reason not to.
Related to #16579 (closed)