tomahawk: Fix finding Phonon VLC backend.

Using propagatedBuildInputs only works for environment.systemPackages
but not for using nix-env, because on NixOS we already have a default
QT_PLUGIN_PATH set there.

The main reason why I'm using the VLC backend and not providing options
for other Phonon backends is because it's recommended upstream and also
will be directly used (via libvlc) in the upcoming 0.9 release.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>