freedreno/rnn: also look for .xml.gz

libxml2 can load gzip compressed files, so lets look for these too.
This will be useful for installing the register database so that an
installed cffdump/crashdec can use them.  But it isn't too useful
to be able to edit the installed register database, so we can gzip
them to use less disk space.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6154>