omniorb: 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/development/tools/omniorb/default.nix

···
   version = "4.2.0";
 
   src = fetchurl rec {
-    url = "http://sourceforge.net/projects/omniorb/files/omniORB/omniORB-${version}/omniORB-${version}.tar.bz2";
+    url = "mirror://sourceforge/project/omniorb/omniORB/omniORB-${version}/omniORB-${version}.tar.bz2";
     sha256 = "1g58xcw4641wyisp9wscrkzaqrz0vf123dgy52qq2a3wk7y77hkl";
   };