Fix compilation issues under gcc11

[Alaettin Serhan Mete]

As reported by Johannes, we seem to have issues building in gcc11:

/build/atnight/localbuilds/nightlies/master--dev4LCG/build/build/AthenaExternals/src/prmon/package/src/registry.h: In instantiation of 'static std::vector<std::__cxx11::basic_string<char> > registry::Registry<T, Pack>::list_registered() [with T = Imonitor; Pack = {}]':
/build/atnight/localbuilds/nightlies/master--dev4LCG/build/build/AthenaExternals/src/prmon/package/src/prmon.cpp:42:60:   required from here
/build/atnight/localbuilds/nightlies/master--dev4LCG/build/build/AthenaExternals/src/prmon/package/src/registry.h:68:21: error: loop variable 'k' creates a copy from type 'const std::pair<const std::__cxx11::basic_string<char>, std::function<Imonitor*()> >' [-Werror=range-loop-construct]

   68 |     for (const auto k : ctors()) {
      |                     ^
/build/atnight/localbuilds/nightlies/master--dev4LCG/build/build/AthenaExternals/src/prmon/package/src/registry.h:68:21: note: use reference type to prevent copying
   68 |     for (const auto k : ctors()) {
      |                     ^
      |                     &

We should go ahead and make sure we can compile w/ gcc11 (it would be nice to extend the CI coverage too).

[Graeme A Stewart]

Closed with #192