This function was always documented to return a sunk reference, but it
did not do so. This change is technically backward incompatible.

However, it only has an effect if anything is doing a g_object_ref_sink.
Which may happen inside libfprint itself. With the change, most API
users (including fprintd) are fixed to do refcounting correctly. Any API
user which worked around this will have a memory leak now.

That is not ideal, but it is not really that bad overall. And returning
a floating reference for FpPrint creation was a bad idea in the first
place. And it really only makes sense for fp_print_new as the only
(public) use case is to create the template for enrollment.