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.

added bugfix label

This merge request affects 1 package:

• Generators/GeneratorObjects

This merge request affects 1 file:

• Generators/GeneratorObjects/test/HepMcParticleLink_test.cxx

added Generators main review-pending-level-1 labels

CI Result SUCCESS (hash fe31724c)

	Athena	AthSimulation	AthGeneration	AthAnalysis
externals
cmake
make
tests

Full details available on this CI monitor view. Check the JIRA CI status board for known problems
Athena: number of compilation errors 0, warnings 0
AthSimulation: number of compilation errors 0, warnings 0
AthGeneration: number of compilation errors 0, warnings 0
AthAnalysis: number of compilation errors 0, warnings 0
For experts only: Jenkins output [CI-MERGE-REQUEST-EL9 5615] (remote access info)

Looks good to me. L1

added review-approved label and removed review-pending-level-1 label

merged

mentioned in commit 3d93c304