nixos/kresd: Fix unsupported regex

Regex supported by `builtins.match` differ on Linux and Darwin
(see NixOS/Nix#1537) and the empty match group errors on Darwin.
But simply removing it does not change the logic in the module in any
way.

nixos/modules/services/networking/kresd.nix

@@ -10,7 +10,7 @@ let
   mkListen = kind: addr: let
     al_v4 = builtins.match "([0-9.]\+):([0-9]\+)" addr;
     al_v6 = builtins.match "\\[(.\+)]:([0-9]\+)" addr;
-    al_portOnly = builtins.match "()([0-9]\+)" addr;
+    al_portOnly = builtins.match "([0-9]\+)" addr;
     al = findFirst (a: a != null)
       (throw "services.kresd.*: incorrect address specification '${addr}'")
       [ al_v4 al_v6 al_portOnly ];