ActsGeantFollower should explicitly depend on Geant4

The ActsGeantFollower package/component explicitly depends on Geant4 as it #includes/uses headers/types from Geant4, but it does not declare this in its CMake script and usage requirements. It currently only compiles via relying on transitive PUBLIC dependencies on other Geant4-using Athena packages.

Find Geant4 in ActsGeantFollower and explicitly use/link to its headers/libraries.

Whilst a small build issue, it will help in integrating ATLASSIM-3150 (Big Library for Simulation) which requires tracking of Athena packages that use/depend on Geant4.