gust_elixir fails to recreate archives with dummy entries

[VitaSmith]

Some archives, such as Ryza 2's pc20c_model.elixir.gz have dummy elixir entries (i.e. 0 sized entries with the name dummy), which we obviously don't extract.

However, because we don't register these entries in the JSON list of extracted files, one may then get an error when recreating the elixir archive:

> gust_elixir.exe pc20c_model
Creating 'pc20c_model.elixir.gz'...
Saved backup as 'pc20c_model.elixir.gz.bak'
ERROR: number of files doesn't match header

gust_elixir should therefore keep the dummy entries listed in the .json and add those dummy entries back to the re-created .elixir[.gz].