build: Enable the use of the dbusxx-xml2cpp tool built from contrib.

Previously, the build machine would need to provide dbusxx-xml2cpp
itself, while the library would be used from the contribs (always
forced, as dbusxx-xml2cpp is unmaintained and slightly broken on newer
GCCs).  This change removes the requirement to have the dbus-c++
package installed on the build machine.

Change-Id: I25ad2ee3e80afc33b320de9256b724a3c254f4fb