Fix HepMcParticleLink_test compilation with HepMC2 (ATLASSIM-7101)
Add HepMC2-specific syntax. Should fix ATLASSIM-7101.
Tested locally, but the CI uses HepMC3, so will not see any change in code compiled.
Add HepMC2-specific syntax. Should fix ATLASSIM-7101.
Tested locally, but the CI uses HepMC3, so will not see any change in code compiled.