xorg.libpciaccess: Not supported on Darwin

http://hydra.nixos.org/build/38160825/nixlog/1/raw

pkgs/servers/x11/xorg/overrides.nix

···
     outputs = [ "dev" "out" ]; # mainly to avoid propagation
   };
 
+  libpciaccess = attrs: attrs // {
+    meta = attrs.meta // { platforms = stdenv.lib.platforms.linux; };
+  };
+
   setxkbmap = attrs: attrs // {
     postInstall =
       ''