Show handshake timestamp in system timezone (opnsense#2204)

This change shows each latest handshake time in the widget in the timezone of the system, rather than UTC as currently. The change is intended to implement this forum request: https://forum.opnsense.org/index.php?topic=20530.msg95463#msg95463.

As a disclaimer, I am by no means a PHP programmer, but this commit is based on my research. It works when I tested it.

net/wireguard/src/www/widgets/widgets/wireguard.widget.php

@@ -55,6 +55,7 @@
         $latest = "-";
         if ($epoch > 0):
             $dt = new DateTime("@$epoch");
+            $dt->setTimezone(new DateTimeZone(date_default_timezone_get()));
             $latest = $dt->format(gettext("Y-m-d H:i:sP"));
         endif; ?>