Motivation for this change

systemd didn't build under gcc8. This suppresses the warnings, my prediction is that this is only needed for this systemd release and that the next release will have fixed this. I have left a note to the next person updating systemd to check this.

This was failing on Hydra (linux-x86_64) due to memory shortage, here's the (linux-aarch64) log:
https://hydra.nixos.org/build/73809444/nixlog/1

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.