nixos/graylog: use types.lines for extraConfig

The `types.lines` type makes it possible to define `extraConfig` in
multiple files and simply concat the contents.

nixos/modules/services/logging/graylog.nix

@@ -108,7 +108,7 @@ in
       };
 
       extraConfig = mkOption {
-        type = types.str;
+        type = types.lines;
         default = "";
         description = "Any other configuration options you might want to add";
       };