fix(subscription) Add missing main.inc file for 1.3-fixes

issue #6152

plugins/addons/subscription/main.inc

+<?php
+/*
+  This code is part of FusionDirectory (http://www.fusiondirectory.org)
+  Copyright (C) 2013-2021  FusionDirectory
+
+  This program is free software; you can redistribute it and/or modify
+  it under the terms of the GNU General Public License as published by
+  the Free Software Foundation; either version 2 of the License, or
+  (at your option) any later version.
+
+  This program is distributed in the hope that it will be useful,
+  but WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+  GNU General Public License for more details.
+
+  You should have received a copy of the GNU General Public License
+  along with this program; if not, write to the Free Software
+  Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301, USA.
+*/
+
+subscriptionInfo::mainInc();