macros: add _nm_fallthrough macro
Systemd introduced a _fallthrough_ macro in https://github.com/systemd/systemd/pull/7389. There might still be some issue with it, but as I am going to re-import the latest systemd code, we get them too. We need it, because "shared/nm-utils/siphash24.c" will use it too, and that source file does not include the other systemd macros. So, we will need to re-define it.
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