download: Add SourceForge load balancer.

* guix/download.scm (%mirrors)[sourceforge]: Add canonical load balancer for
sourceforge.net.

Signed-off-by: Leo Famulari <leo@famulari.name>

guix/download.scm

@@ -2,6 +2,7 @@
 ;;; Copyright © 2012, 2013, 2014, 2015, 2016 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2013, 2014, 2015 Andreas Enge <andreas@enge.fr>
 ;;; Copyright © 2015 Federico Beffa <beffa@fbengineering.ch>
+;;; Copyright © 2016 Alex Griffin <a@ajgrf.com>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -99,6 +100,7 @@ (define %mirrors
        "http://www.centervenus.com/mirrors/nongnu/"
        "http://download.savannah.gnu.org/releases-noredirect/")
       (sourceforge ; https://sourceforge.net/p/forge/documentation/Mirrors/
+       "http://downloads.sourceforge.net/project/"
        "http://ufpr.dl.sourceforge.net/project/"
        "http://heanet.dl.sourceforge.net/project/"
        "http://freefr.dl.sourceforge.net/project/"