libostree: Add _finish() API to async progress

Since OstreeAsyncProgress queues to the mainloop, we might "lose" the
last message.  Give callers a way to force a flush.

src/libostree/ostree-async-progress.h

@@ -64,5 +64,7 @@ void ostree_async_progress_set_uint64 (OstreeAsyncProgress       *self,
                                        const char                *key,
                                        guint64                    value);
 
+void ostree_async_progress_finish (OstreeAsyncProgress *self);
+
 G_END_DECLS