cups: add systemd to build inputs on linux

Otherwise, cups will be built without systemd support. The upstream
systemd service unit expects cups to have systemd support however. When
cups is built without systemd support, the daemon will fork to the
background. Consequently, systemd will think the service is done and
enter inactive state.

Along with 9de93be6 and
47330b17 this patch fixes the nixos
printer test timing out in `waitForUnit("cups.service")`; see
NixOS/nixpkgs#14748