tomahawk: mark as broken

During `8d6502f8ca9` `phonon-backend-vlc` uses QT5 by default.
Rather than fixing this it has been marked as broken as it's currently
unmaintained (see c76ad476b0)
and broken since March (https://hydra.nixos.org/build/71873808).
For now I've marked it as broken as the last *stable* release is from
2015 (see https://github.com/tomahawk-player/tomahawk/releases/tag/0.8.4).

pkgs/applications/audio/tomahawk/default.nix

@@ -51,5 +51,6 @@ stdenv.mkDerivation rec {
     homepage = http://tomahawk-player.org/;
     license = licenses.gpl3Plus;
     platforms = platforms.all;
+    broken = true; # 2018-06-25
   };
 }