gtkmodule: Preserve order of required-gtype-classes for reproducibility

The elements of current_required_app_classes end up in the generated
code in g_type_ensure() statements. The order of these can vary
between builds when using a HashSet, breaking reproducibilty.

See https://wiki.debian.org/ReproducibleBuilds

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=802528