src/rss_converter_*.xsl: add missing generate-enclosure parameter

src/rss_converter_*.xsl: add missing generate-enclosure parameter

XSL parameters do not necessarily need to be declared in the stylesheet
if no default value is explicitly set, however tweeper is doing that for
other stylesheet, so declare the parameter in rss_converter_pump.io.xsl
and rss_converter_dilbert.com.xsl as well for consistency.