Add GenCrossSection to AtlasHepMC

• Add GenCrossSection to AtlasHepMC
• Use "AtlasHepMC/GenCrossSection.h" instead of #ifdef

Tag @jchapman

Generators/AtlasHepMC/AtlasHepMC/GenCrossSection.h

+/*
+  Copyright (C) 2002-2022 CERN for the benefit of the ATLAS collaboration
+*/
+/* Author: Andrii Verbytskyi andrii.verbytskyi@mpp.mpg.de */
+
+#ifndef ATLASHEPMC_CROSSECTION_H
+#define ATLASHEPMC_CROSSECTION_H
+#ifdef HEPMC3
+#include "HepMC3/GenCrossSection.h"
+#else
+#include "HepMC/GenCrossSection.h"
+#endif
+#endif