syntax: xml: recognize an XML file also by its first line

Some .rdf files are XML, but the .rdf extension seems to be used
also for other things.  So... use the 'header' mechanism.

syntax/xml.nanorc

@@ -1,6 +1,7 @@
 ## Here is an example for XML files.
 
 syntax "xml" "\.([jrsx]html?|jnlp|mml|pom|rng|sgml?|svg|w[as]dl|wsdd|xjb|xml|xs(d|lt?)|xul)$"
+header "<\?xml.*version=.*\?>"
 magic "(XML|SGML) (sub)?document text"
 comment "<!--|-->"