Fix prematually closed attribute

[onacit]

It seems that an xs:attribute prematurely closed itself.

See
https://github.com/xsf/schemas/blob/master/jingle-apps-dtls.xsd#L21-L35

AS-IS

          <xs:attribute name='setup' use='required'/> <!-- shouldn't close itself -->
            <xs:simpleType>
              <xs:restriction base='xs:NCName'>
                <xs:enumeration value='active'/>
                <xs:enumeration value='passive'/>
                <xs:enumeration value='actpass'/>
                <xs:enumeration value='holdconn'/>
                <xs:annotation>
                  <xs:documentation>
                    the 'holdconn' value is not used and included only for completeness.
                  </xs:documentation>
                </xs:annotation>
              </xs:restriction>
            </xs:simpleType>
          </xs:attribute>

TO-BE

          <xs:attribute name='setup' use='required'>
            <xs:simpleType>
              <xs:restriction base='xs:NCName'>
                <xs:enumeration value='active'/>
                <xs:enumeration value='passive'/>
                <xs:enumeration value='actpass'/>
                <xs:enumeration value='holdconn'/>
                <xs:annotation>
                  <xs:documentation>
                    the 'holdconn' value is not used and included only for completeness.
                  </xs:documentation>
                </xs:annotation>
              </xs:restriction>
            </xs:simpleType>
          </xs:attribute>