serivces: service probe for dhcpv6 causes missing deps; closes #1065

This solves the missing dependencies problem after backporting
21f651c6 in a somewhat crude way, though 16.1.20 is EOL it won't
matter much.

Suggested by: @devste

src/etc/inc/services.inc

@@ -1057,6 +1057,7 @@ function is_dhcpv6_server_enabled()
     if (is_array($config['interfaces'])) {
         foreach ($config['interfaces'] as $ifcfg) {
             if (isset($ifcfg['enable']) && !empty($ifcfg['track6-interface'])) {
+                require_once("pfsense-utils.inc");
                 $pdlen = calculate_ipv6_delegation_length($ifcfg['track6-interface']);
                 if ($pdlen > 2) {
                     return true;