Fix GlobalSimulation compilation in dbg build

Follow up to !83734 (merged) FYI @peter

Uses constexpr as in https://gitlab.cern.ch/atlas/athena/-/blob/main/Trigger/TrigT1/Global/GlobalSimulation/src/IO/ICommonTOB.h?ref_type=heads#L35 to fix compilation error observed in debug build

https://atlas-sw-build-logs.web.cern.ch/nightlywww/DEVELOPMENTWebArea/ardoc_web_areamain64BE9G14AthenaDbg/ARDOC_Log_2025-10-27T2100/Trigger_TrigT1_Global_GlobalSimulation.html#end

/build/atnight/localbuilds/nightlies/Athena/main/athena/Trigger/TrigT1/Global/GlobalSimulation/src/GlobalAlgs/FirstChain/eEmMultTestBench.cxx:134: undefined reference to `GlobalSim::IOBitwise::IeEmTOB::s_RHad_width'