mldonkey: adds license information

pkgs/applications/networking/p2p/mldonkey/default.nix

···
   meta = {
     description = "Client for many p2p networks, with multiple frontends";
     homepage = http://mldonkey.sourceforge.net/;
+    license = stdenv.lib.licenses.gpl2;
   };
 
   buildInputs = [ ocaml zlib ncurses bzip2 file gd libpng ];