Export gtk-primary-selection-client-protocol headers
Firefox needs to implement wayland clipboard directly due to sync/async clipboard difference in js/Gtk+. Firefox also wants to use the middle selection clipboard but we need to export gtk-primary-selection-client-protocol headers to use it at Firefox code.
It also means to make gtk_primary interfaces public/exported by Gtk+ libs:
struct gtk_primary_selection_device; struct gtk_primary_selection_device_manager; struct gtk_primary_selection_offer;