libtracker-direct: Add missing 'unowned' annotation
The .get_data_manager() method doesn't ref the data manager before returning it. Vala code that called this method wasn't reffing it either because 'owned' is the default ownership flag.
This fixes a critical that sometimes appeared on shutdown of tracker-store:
(tracker-store:5986): GLib-GObject-CRITICAL **: 10:53:13.275: g_object_unref: assertion 'G_IS_OBJECT (object)' failed