wmsystemtray: switch sf downloads to mirror://

This allows fallback to all configured mirrors for sourceforge
to download, instead of relying on just downloads.sourceforge.net

pkgs/applications/window-managers/windowmaker/dockapps/wmsystemtray.nix

···
 stdenv.mkDerivation {
   name = "wmsystemtray-1.4";
   src = fetchurl {
-     url = http://sourceforge.net/projects/wmsystemtray/files/wmsystemtray/wmsystemtray-1.4.tar.gz;
+     url = mirror://sourceforge/project/wmsystemtray/wmsystemtray/wmsystemtray-1.4.tar.gz;
      sha256 = "8edef43691e9fff071000e29166c7c1ad420c0956e9068151061e881c8ac97e9";
   };