phonon-backend-vlc: use qt5 by default

`minitube` is currently broken transitively due to the broken
`phonon-backend-qt4`: https://hydra.nixos.org/build/76523277

Although QT4 is fairly old, this package is still built with `qt4` ATM,
however QT5 is available as well. After this change, QT5 will be built
by default and in case anybody requires legacy QT4 it has to be enabled
explicitly like this:

```
with import <nixpkgs> { };
phonon-backend-vlc.override { withQt4 = true; }
```

Now the QT5-only build can be used (which fixes `minitube`) and there
are no confusions anymore with the build dependencies. Previously
`phonon-backend-vlc` and `libsForQt5.phonon-backend-vlc` used `qt4` by
default which was likely responsible for broken `minitube`.