fix(elem.metamark): allow xml:id in metamark

schema/edxml.rng

@@ -503,6 +503,9 @@
                     </sch:rule>
                 </sch:pattern>
             </attribute>
+            <optional>
+                <ref name="attr.xml.id"/>
+            </optional>
             <empty/>
         </element>
     </define>