Fix FlowAfterburner compilation in master--HepMC2
This fix only affects the master--HepMC2
nightlies, so the code change will not be seen by the CI (which is HepMC3-based like the master nightlies).
This fix only affects the master--HepMC2
nightlies, so the code change will not be seen by the CI (which is HepMC3-based like the master nightlies).