backend: Export the enumeration type definitions
backend: Export the enumeration type definitions

Export the GType definition for the GoaProviderGroup enumeration to
be able to box it inside GValue, etc.

https://bugzilla.gnome.org/show_bug.cgi?id=695886

ritzk/gnome-online-accounts,dnohales/gnome-online-accounts,felipeborges/gnome-online-accounts,halfline/gnome-online-accounts,ritzk/gnome-online-accounts,Distrotech/gnome-online-accounts,ritzk/gnome-online-accounts,halfline/gnome-online-accounts,dnohales/gnome-online-accounts,halfline/gnome-online-accounts,felipeborges/gnome-online-accounts,Distrotech/gnome-online-accounts,dnohales/gnome-online-accounts,Distrotech/gnome-online-accounts,felipeborges/gnome-online-accounts