comparators: xml: also recognize "GnuCash file" as XML

file says:

% file foo.gnucash
foo.gnucash: GnuCash file

gnucash files are, by default, gz-compressed, and file recognizes them
as such:

% file main.gnucash
main.gnucash: gzip compressed data, from Unix, original size modulo 2^32 1330067

But once uncompressed they are normal XML files, but somehow `file` has
its own filetype for them.

Signed-off-by: Mattia Rizzolo <mattia@debian.org>