polkit: Don't build elogind on non-Linux.
polkit: Don't build elogind on non-Linux.

In #72057 I added support for elogind when systemd is not supported,
but it occurred to me that it probably doens't make sense to have
either of systemd or elogind on Darwin.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs