Revert "registrybackend: use G_DECLARE_FINAL_TYPE"
Revert "registrybackend: use G_DECLARE_FINAL_TYPE"

This reverts commit b0776ddd1850f5ca91fe50c11311fe5c9c90cf53.
mingw does not seem to like this patch so better go on the safe
way.

johne53/MB3Glib,endlessm/glib,endlessm/glib,MathieuDuponchelle/glib,MathieuDuponchelle/glib,mzabaluev/glib,johne53/MB3Glib,mzabaluev/glib,Distrotech/glib,endlessm/glib,endlessm/glib,Distrotech/glib,mzabaluev/glib,MathieuDuponchelle/glib,johne53/MB3Glib,johne53/MB3Glib,MathieuDuponchelle/glib,mzabaluev/glib,MathieuDuponchelle/glib,mzabaluev/glib,Distrotech/glib,johne53/MB3Glib,endlessm/glib,Distrotech/glib,Distrotech/glib,johne53/MB3Glib