MAX_BCID is not constexpr: fix clang build

Fix clang build for LumiBlockComps - size() of an input parameter is not constexpr.

See build failure here: http://atlas-computing.web.cern.ch/atlas-computing/links/distDirectory/gitwww/GITWebArea/nightlies/master/2019-11-10T2108/Athena/x86_64-centos7-clang8-opt/Athena/LumiBlock.LumiBlockComps.log.html

cc @ssnyder so our merge requests don't clash (or clang ;-)