girepository: Make GIInfoType private

Eventually, we want to move to using `GType` directly for everything,
since `GIBaseInfo` and its subclasses are all using `GTypeInstance`.

However, that requires quite a lot of changes and we’re about to hit the
API freeze.

So do the smallest set of changes possible to remove `GIInfoType` and
related functions from the public API, which gives us freedom to make
more changes later without breaking API.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

Helps: #3155

girepository/girffi.h

@@ -92,7 +92,7 @@ void          gi_type_info_extract_ffi_return_value (GITypeInfo
 
 GI_AVAILABLE_IN_ALL
 void          gi_type_tag_extract_ffi_return_value (GITypeTag         return_tag,
-                                                    GIInfoType        interface_type,
+                                                    GType             interface_type,
                                                     GIFFIReturnValue *ffi_value,
                                                     GIArgument       *arg);