okular: add runtime depend phonon-backend-vlc, required for embedded …

[Luciogi]

…file

Testing the changes

• I tested the changes in this PR: YES|briefly|NO

[sgn]

So it's optional dependency, no? What is okular's behaviour when it's not available? Anyway, it's preferable to use virtual?phonon-qt6-backend

[Luciogi]

So it's optional dependency, no? What is okular's behaviour when it's not available? Anyway, it's preferable to use virtual?phonon-qt6-backend

For use case, when audio is embedded in pdf file, it does not play and give errors in terminal output "phonon backend plugin could not be loaded"

[sgn]

I think it's optional dependency. I tend to ignore those optional dependencies, though.