Systemd instroduces a macro _fallthrough_, see
https://github.com/systemd/systemd/pull/7389.
However, it does not yet seem conclusive how to
handle this properly in ever situation.

While shared/nm-utils/siphash24.c makes use of
the new macro, don't do that in our fork. siphash24.h
does not include all systemd headers, hence _fallthrough_
is not defined. We could re-implement it as _nm_fallthrough,
but given the open questions, that doesn't seem the