dd4hep build warning

See https://lhcb-nightlies.web.cern.ch/nightly/lhcb-master/1717/Allen/x86_64_v2-centos7-gcc11%2Bdd4hep-opt/build

/cvmfs/lhcb.cern.ch/lib/lcg/releases/DD4hep/01.19-e2ca1/x86_64-centos7-gcc11-opt/include/DD4hep/Grammar.h:116:57: warning: 'static const dd4hep::BasicGrammar& dd4hep::BasicGrammar::instance() [with TYPE = {anonymous}::MuonTable_t]' used but never defined
   template <typename TYPE> static const BasicGrammar& instance();