xmlrpc: prevent undefined errors on remote service restart

Further proof that all of the system should use the plugin system,
as the dependencies are going to be unmanageable in the long run.
As soon as there is a new base service with its own include file
its definition will be missing.

src/etc/inc/xmlrpc/service.inc

 <?php
+
 /**
  *    Copyright (C) 2016 Deciso B.V.
  *
@@ -26,8 +27,17 @@
  *    POSSIBILITY OF SUCH DAMAGE.
  *
  */
+
 require_once("services.inc");
 require_once("system.inc");
+require_once('plugins.inc');
+require_once("vslb.inc");
+require_once("unbound.inc");
+require_once("openvpn.inc");
+require_once("filter.inc");
+require_once("ipsec.inc");
+require_once("interfaces.inc");
+require_once("rrd.inc");
 
 /**
  * @return array