firmware: add new mirror courtsey of http://dmcnet.net/

src/opnsense/mvc/app/controllers/OPNsense/Core/Api/FirmwareController.php

@@ -582,6 +582,7 @@ class FirmwareController extends ApiControllerBase
         $mirrors['https://opnsense-update.deciso.com'] = 'Deciso (NL, Commercial)';
         $mirrors['https://mirror.auf-feindgebiet.de/opnsense'] = 'auf-feindgebiet.de (Cloudflare CDN)';
         $mirrors['https://opnsense.c0urier.net'] = 'c0urier.net (Lund, SE)';
+        $mirrors['http://mirrors.dmcnet.net/opnsense'] = 'DMC Networks (Lincoln NE, US)';
         //$mirrors['https://fleximus.org/mirror/opnsense'] = 'Fleximus (Roubaix, FR)';
         $mirrors['https://fourdots.com/mirror/OPNSense'] = 'FourDots (Belgrade, RS)';
         $mirrors['http://mirror.ams1.nl.leaseweb.net/opnsense'] = 'LeaseWeb (Amsterdam, NL)';