Update en_system_config.lng

Author: martin-enavn

interface/web/admin/lib/lang/en_system_config.lng

@@ -55,6 +55,7 @@ $wb['backups_include_into_web_quota_txt'] = 'Include backup files into web quota
 $wb['mailbox_show_autoresponder_tab_txt'] = 'Show autoresponder tab in mail account details';
 $wb['mailbox_show_mail_filter_tab_txt'] = 'Show mail filter tab in mail account details';
 $wb['mailbox_show_custom_rules_tab_txt'] = 'Show custom mailfilter tab in mail account details';
+$wb['mailbox_show_mail_backup_txt'] = 'Show backup tab in mail account details';
 $wb['webmail_url_error_regex'] = 'Invalid webmail URL';
 $wb['phpmyadmin_url_note_txt'] = 'Placeholder:';
 $wb['webmail_url_note_txt'] = 'Placeholder:';