services: fix a crash report related to calculate_ipv6_delegation_length()

(cherry picked from commit 77578606)

src/etc/inc/xmlrpc/service.inc

@@ -32,6 +32,7 @@ require_once("services.inc");
 require_once("system.inc");
 require_once('plugins.inc');
 require_once("vslb.inc");
+require_once('pfsense-utils.inc');
 require_once("unbound.inc");
 require_once("openvpn.inc");
 require_once("filter.inc");