Add 'xz' to valid compress values

As reported in issue #226 the 'xz' compressor was missing.

Author: sebastianfeldmann

phpbu.xsd

@@ -73,6 +73,7 @@
           <xs:enumeration value="gzip"/>
           <xs:enumeration value="zip"/>
           <xs:enumeration value="7zip"/>
+          <xs:enumeration value="xz"/>
         </xs:restriction>
       </xs:simpleType>
     </xs:attribute>