Compilation fixes for the main--HepMC2 nightly
As pointed out in ATLASSIM-6994, there are build warnings in the MCTruth package in the main--HepMC2. These are fixed in the first commit.
The second commit fixes the compilation failure of SegmentDriftCircleAssValidation
in the main--HepMC2 nightlies.
Tagging @averbyts