tomahawk: mark as broken

During `8d6502f8` `phonon-backend-vlc` uses QT5 by default.
Rather than fixing this it has been marked as broken as it's currently
unmaintained (see c76ad476b0245c474c057b451ba6fec18e7f81f1)
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).