MINOR: sink: Add a ref to sink in the sink_forward_target structure

This change is required to be able to refactor the init stage of appctx. It
is now possible to directly get the sink from a forward target.
diff --git a/src/sink.c b/src/sink.c
index 5939185..681f363 100644
--- a/src/sink.c
+++ b/src/sink.c
@@ -1005,6 +1005,7 @@
 	/* insert into sink_forward_targets
 	 * list into sink
 	 */
+	sft->sink = sink;
 	sft->next = sink->sft;
 	sink->sft = sft;