Geant4Output2EDM4hep: fix "non-trivial type in variadic function

/build/jenkins/workspace/lcg_nightly_pipeline/build/frameworks/DD4hep-01.25/src/DD4hep/01.25/DDG4/edm4hep/Geant4Output2EDM4hep.cpp:100:80: error: cannot pass object of non-trivial type 'const std::basic_string<char>' through variadic function; call will abort at runtime [-Wnon-pod-varargs]
        printout(DEBUG, "Geant4OutputEDM4hep", "Saving event parameter: %s", p.first);
                                                                               ^
/build/jenkins/workspace/lcg_nightly_pipeline/build/frameworks/DD4hep-01.25/src/DD4hep/01.25/DDG4/edm4hep/Geant4Output2EDM4hep.cpp:104:80: error: cannot pass object of non-trivial type 'const std::basic_string<char>' through variadic function; call will abort at runtime [-Wnon-pod-varargs]
        printout(DEBUG, "Geant4OutputEDM4hep", "Saving event parameter: %s", p.first);
                                                                               ^
/build/jenkins/workspace/lcg_nightly_pipeline/build/frameworks/DD4hep-01.25/src/DD4hep/01.25/DDG4/edm4hep/Geant4Output2EDM4hep.cpp:108:80: error: cannot pass object of non-trivial type 'const std::basic_string<char>' through variadic function; call will abort at runtime [-Wnon-pod-varargs]
        printout(DEBUG, "Geant4OutputEDM4hep", "Saving event parameter: %s", p.first);