add a missed option to admin plugin

plugins/administrator/defines.php

@@ -198,6 +198,8 @@ $defcfg = array( '$config_version' => array( 'name' => _("Config File Version"),
                                                     'type' => SMOPT_TYPE_BOOLEAN ),
                  '$hide_sm_attributions' => array( 'name' => _("Hide SM attributions"),
                                                    'type' => SMOPT_TYPE_BOOLEAN ),
+                 'default_use_mdn' => array( 'name' => _("Enable use of delivery receipts"),
+                                             'type' => SMOPT_TYPE_BOOLEAN ),
                  '$edit_identity' => array( 'name' => _("Allow editing of identities"),
                                             'type' => SMOPT_TYPE_BOOLEAN ),
                  '$edit_name' => array( 'name' => _("Allow editing of full name"),