freedv: mark as broken on darwin

freedv doesn't build on darwin, due to missing dependencies. Building it
on darwin is an elaborate process, judging by it's `build_osx.sh`
script, that requires fetching some additional dependencies.

pkgs/applications/radio/freedv/default.nix

@@ -48,5 +48,6 @@ stdenv.mkDerivation rec {
     license = licenses.lgpl21;
     maintainers = with maintainers; [ mvs ];
     platforms = platforms.unix;
+    broken = stdenv.isDarwin;  # see https://github.com/NixOS/nixpkgs/issues/165422
   };
 }