Fix rebinding of GntBindables.

`gnt_bindable_register_binding` takes a variadic set of arguments, but
when rebinding, we only have the condensed `GList`. Instead, use the
internal `register_binding` function that *does* take a `GList`.

Fortunately, no previous calls to `gnt_bindable_register_binding` were
ever passed parameters, so this would always copy an empty list anyway.