fix clang11 warnings

See e.g.

https://lhcb-nightlies.web.cern.ch/nightly/lhcb-master/1398/LHCb/x86_64-centos7-clang11-opt/build#Kernel/SOAContainer/submodules/SOAContainer/examples_5

tested with clang11 and gcc10

@mschille please could this be merged soon, so we can then update the LHCb submodule configuration to squash the warnings there.