Add smart pointers for FlowElementContainer

Adding missing line in CMakeList.txt

so

cat /cvmfs/atlas-nightlies.cern.ch/repo/sw/master_Athena_x86_64-centos7-gcc8-opt/2020-09-17T2101/Athena/22.0.18/InstallArea/x86_64-centos7-gcc8-opt/lib/Athena.rootmap | grep FlowElementContainer_v1
class xAOD::FlowElementContainer_v1

vs

cat x86_64-centos7-gcc8-opt/lib/WorkDir.rootmap |grep FlowElementContainer_v1
class DataLink<xAOD::FlowElementContainer_v1>
class ElementLink<xAOD::FlowElementContainer_v1>
class SG::AuxTypeVectorFactory<DataLink<xAOD::FlowElementContainer_v1> >
class SG::AuxTypeVectorFactory<ElementLink<xAOD::FlowElementContainer_v1> >
class SG::AuxTypeVectorFactory<std::vector<ElementLink<xAOD::FlowElementContainer_v1> > >
class vector<DataLink<xAOD::FlowElementContainer_v1> >
class vector<ElementLink<xAOD::FlowElementContainer_v1> >
class vector<std::vector<ElementLink<xAOD::FlowElementContainer_v1> > >
class xAOD::FlowElementContainer_v1

Mentioning @mhodgkin , @manthony , @ssnyder ,@akraszna

Event/xAOD/xAODPFlow/CMakeLists.txt

@@ -18,7 +18,7 @@ atlas_add_library( xAODPFlow
 atlas_add_xaod_smart_pointer_dicts(
    INPUT xAODPFlow/selection.xml
    OUTPUT _selectionFile
-   CONTAINERS "xAOD::PFOContainer_v1" "xAOD::TrackCaloClusterContainer_v1" )
+   CONTAINERS "xAOD::PFOContainer_v1" "xAOD::TrackCaloClusterContainer_v1"  "xAOD::FlowElementContainer_v1")
 
 atlas_add_dictionary( xAODPFlowDict
    xAODPFlow/xAODPFlowDict.h