rss_converter_*.xsl: specify xml:base

Some feed readers expand relative URLs in items by extracting the base
URL from the enclosing <link/> element, however this is not a globally
accepted practice.

Specifying xml:base is useful to cover other ways in which relative URLs
are expanded, see:
  https://cyber.law.harvard.edu/rss/relativeURI.html
  http://www.w3.org/TR/xmlbase/#resolution