BUG/MINOR: sink: missing sft free in sink_deinit()

Adding missing free for sft (string_forward_target) in sink_deinit(),
which resulted in minor leak for each declared ring target at deinit().
(either explicit and implicit rings are affected)

This may be backported up to 2.4.

src/sink.c

@@ -1370,6 +1370,7 @@ static void sink_deinit()
 		free_proxy(sink->forward_px);
 		free(sink->name);
 		free(sink->desc);
+		free(sink->sft);
 		free(sink);
 	}
 }