qdbusxml2cpp: Use the mtime on the input XML to avoid needless source changes.

These can cause rebuilds unnecessarily when repeatedly running qdbusxml2cpp.

Change-Id: I902954d4bed6fe68802183e51d82700fe30af437
Reviewed-by: Mathias Hasselmann <mathias.hasselmann@kdab.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>