BUG/MINOR: http-rules: Mark http return rules as final

For HTTP rules, this flag is only used to trigger a warning during HAProxy
startup when a final rule without ACL is not the last one. So this patch is
marked as a bug, but its impact is really limited.

No backport needed because http return rules were introduced in the 2.2.
diff --git a/src/http_act.c b/src/http_act.c
index e6e3df4..66cbe13 100644
--- a/src/http_act.c
+++ b/src/http_act.c
@@ -2417,6 +2417,7 @@
 		free(obj);
 	}
 
+	rule->flags |= ACT_FLAG_FINAL;
 	rule->arg.http_return.status = status;
 	rule->arg.http_return.ctype = ctype;
 	rule->arg.http_return.hdrs = hdrs;