Add missing header to GaudiKernel/DataSvc.h

Some packages in the ATLAS offline code started failing because of this
missing include. Since the SmartIF<IConversionSvc> member variable does
need a full declaration of this type.

Fixes GAUDI-1163.
See merge request !90

GaudiKernel/GaudiKernel/DataSvc.h

@@ -8,6 +8,7 @@
 #include "GaudiKernel/IDataProviderSvc.h"
 #include "GaudiKernel/IIncidentSvc.h"
 #include "GaudiKernel/SmartIF.h"
+#include "GaudiKernel/IConversionSvc.h"
 
 #include <boost/utility/string_ref.hpp>