Send the siril.xml file to the right freedesktop dir

Well, it seems I was wrong about the mime file. Found this: https://specifications.freedesktop.org/shared-mime-info-spec/shared-mime-info-spec-latest.html

It tells us that the applications mime file should be installed in a subdir of /usr/share/mime. @Vincent-FA seems to be using /usr/share/mime/packages for the AUR package on Archlinux, just like Debian, Ubuntu and Fedora.

If that's seems good enough, this merge request adds the installation of the mime data to /usr/share/mime/packages