Let non-important files be listed in needed files on dedicated servers

Right now, when adding files that only contains audio lumps, the files is marked as non-important and not added to the list of required files. This became an issue for me when setting up my server to run Sonic 2006 with music, since the music (and nothing else) has been seperated into a PK3 of it's own and is thus not loaded by clients when they join the server. It's also generally confusing behavior that specifying some PK3s on a dedicated server effectively becomes a no-op if it's only music in them.

Therefore, I think it's a better to allow PK3s to be listed on dedicated servers regardless of what they contain, because as of right now, there's no way to mark these files for transfer on a server. Whenever this is a good idea or not is still up for debate, though.