types: fix various use of volatile in type registration

types: fix various use of volatile in type registration

The use of volatile was incorrect in GLib and has been that way for
a long time. Recently however that has changed, and this makes GTK
follow suit to avoid using volatile in the type registration.

See also: https://gitlab.gnome.org/GNOME/glib/-/merge_requests/1719

Combined with the above merge request for GLib, this fixes a large
number of compilation warnings when using Clang.