Ardour: Add package dependencies for mp3 export

[jbmestelan]

I found it necessary to install these packages, so that Ardour can export to mp3 format:

ffmpeg
harvid

Would you consider adding these as dependencies (or recommended dependencies)?

ref: https://discourse.ardour.org/t/mp3-export-encoder-not-available/103998/7

[ycollet]

The only problem I see is that I am not managing the ardour package. ardour is still part of the fedora main repository.
And if you open a ticket on fedora, they will not have "access" to harvid which is a package of my repo.
I will check if it's possible to find an intermediate solution (like moving harvid to main fedora repository).