Add variable DD4hep_SOVERSION in client cmake config.

cmake/DD4hepConfig.cmake.in

@@ -16,6 +16,7 @@
 set ( DD4hep_DIR         "$ENV{DD4hep_DIR}" )
 set ( DD4hep_ROOT        "$ENV{DD4hep_ROOT}" )
 set ( DD4hep_VERSION     "@DD4hep_VERSION@" )
+set ( DD4hep_SOVERSION   "@DD4hep_SOVERSION@" )
 set ( DD4HEP_USE_BOOST   "@DD4HEP_USE_BOOST@" )
 set ( DD4HEP_USE_GEANT4  "@DD4HEP_USE_GEANT4@" )
 set ( DD4HEP_USE_XERCESC "@DD4HEP_USE_XERCESC@" )