xep.xsl: use proper biblatex name list part ordering

Name lists start with the family name, follow by the given name. See
biblatex(.pdf) § 2.2.1.

xep.xsl

@@ -509,7 +509,7 @@ content: "XEP-<xsl:value-of select='/xep/header/number'/>";
   version = {<xsl:value-of select='/xep/header/revision[position()=1]/version'/>},
   type = {XEP},
   number = {<xsl:value-of select='/xep/header/number'/>},
-  author = {<xsl:for-each select='/xep/header/author'><xsl:value-of select='firstname'/><xsl:text> </xsl:text><xsl:value-of select='surname'/><xsl:if test="not(position() = last())"> and </xsl:if></xsl:for-each>},
+  author = {<xsl:for-each select='/xep/header/author'><xsl:value-of select='surname'/><xsl:text>, </xsl:text><xsl:value-of select='firstname'/><xsl:if test="not(position() = last())"> and </xsl:if></xsl:for-each>},
   institution = {XMPP Standards Foundation},
   url = {https://xmpp.org/extensions/xep-<xsl:value-of select='/xep/header/number'/>.html},
   date = {<xsl:value-of select='/xep/header/revision[position()=last()]/date'/>/<xsl:value-of select='/xep/header/revision[position()=1]/date'/>},