Fix debug build warning in H5Utils

Addresses https://atlas-sw-build-logs.web.cern.ch/nightlywww/DEVELOPMENTWebArea/ardoc_web_areamain64BE9G13AthenaDbg/ARDOC_Log_2024-11-06T2101/PhysicsAnalysis_AnalysisCommon_HDF5Utils.html

/build/atnight/localbuilds/nightlies/Athena/main/athena/PhysicsAnalysis/AnalysisCommon/HDF5Utils/HDF5Utils/histogram.icc:220:39: warning: comparison of integer expressions of different signedness: 'hssize_t' {aka 'long int'} and 'std::vector<double, std::allocator<double> >::size_type' {aka 'long unsigned int'} [-Wsign-compare]
  220 |       assert(space.getSelectNpoints() == flat.size());