Fix sTGCSimHitVariable.cxx for HepMC3 builds
One line in sTGCSimHitVariable was relying on implict type conversion
from double to HepMC::ThreeVector
which is not available in HepMC3-based builds.
(The constructor used is HepMC::ThreeVector(double xin, double yin=0, double zin=0)
.)
Pushing this change to 22.0 initially just to prevent any future conflicts when sweeping.
Tagging @jojungge for info.