codegen: Add g_autoptr support for the shared GInterface

The rest of the generated classes gained g_autoptr support in fd6ca66c

,
but this one is still missing. Because whatever_proxy_new_finish() and
whatever_proxy_new_sync() are declared as returning a Whatever *
instead of a WhateverProxy *, and the generated method-call stubs
act on a Whatever *, it's reasonably common to want to declare a
g_autoptr (Whatever).

Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Bug: https://bugzilla.gnome.org/review?bug=763379


Reviewed-by: Colin Walters <walters@verbum.org>