makeDBusConf: make apparmor argument optional

The only in-tree use of this is the DBus NixOS module, which always
passes apparmor (but the services.dbus.apparmor option defaults to
"disabled").  Set the default in the function as well so other users
of the function can take advantage of it.

pkgs/top-level/all-packages.nix

@@ -14720,7 +14720,7 @@ in
   dbus-sharp-glib-1_0 = callPackage ../development/libraries/dbus-sharp-glib/dbus-sharp-glib-1.0.nix { };
   dbus-sharp-glib-2_0 = callPackage ../development/libraries/dbus-sharp-glib { };
 
-  makeDBusConf = { suidHelper, serviceDirectories, apparmor }:
+  makeDBusConf = { suidHelper, serviceDirectories, apparmor ? "disabled" }:
     callPackage ../development/libraries/dbus/make-dbus-conf.nix {
       inherit suidHelper serviceDirectories apparmor;
     };