Compatibility with libmagic-5.19

This adds compatibility with libmagic-5.19 (and 5.18 still seems to work).

The changes upstream can be seen here: https://github.com/file/file/commit/07bdb1e0d5f91d4efbe718024cbed7a067294446#diff-52

While it seems to work, will it work properly with 5.18?