dnscrypt-proxy: fix faulty use of platforms.allBut

In 63b6498aa0 I added a faulty
use of `platforms.allBut` causing dnscrypt-proxy to continue
being built (and failing) on OS X. D'oh!

pkgs/tools/networking/dnscrypt-proxy/default.nix

@@ -25,6 +25,6 @@ stdenv.mkDerivation rec {
     license = licenses.isc;
     maintainers = with maintainers; [ joachifm jgeerds ];
     # upstream claims OSX support, but Hydra fails
-    platforms = with platforms; allBut [ darwin ];
+    platforms = with platforms; allBut darwin;
   };
 }