[relax-ng] Another bug in relaxng.rng

There's another bug in relaxng.rng: it doesn't allow ref and parentRef to
have foreign child elements.  The fix is:

***************
*** 72,83 ****
--- 72,85 ----
            <data type="NCName"/>
          </attribute>
          <ref name="common-atts"/>
+         <ref name="other"/>
        </element>
        <element name="parentRef">
          <attribute name="name">
            <data type="NCName"/>
          </attribute>
          <ref name="common-atts"/>
+         <ref name="other"/>
        </element>
        <element name="empty">
          <ref name="common-atts"/>