gtk-gnutella: mark as "broken"

The current version doesn't compile with the latest GCC any more.

pkgs/tools/networking/p2p/gtk-gnutella/default.nix

···
     homepage = "http://gtk-gnutella.sourceforge.net/";
     description = "a server/client for Gnutella";
     license = stdenv.lib.licenses.gpl2;
-    platforms = stdenv.lib.platforms.linux;
-    maintainers = [ stdenv.lib.maintainers.simons ];
+    broken = true;
   };
 }