AutoarExtractor: Fix decompressing archive with legacy encoding filenames

The archive in question contains filenames in IBM 437 encoding, likely
generated with an older version of Windows. Convert the filenames to
UTF-8 before writing the files out to disk to avoid "Invalid argument"
errors for filesystems which don't like non-UTF-8 encodings.

https://bugzilla.gnome.org/show_bug.cgi?id=785077