Fix OBJECT_TYPE/OBJECT_DOMAIN confusion

This doesn't have a significant impact except that now SECURITY LABEL ON
DOMAIN rejects types that are not domains.

Reported-by: 高增琦 <pgf00a@gmail.com>

src/backend/parser/gram.y

@@ -6507,7 +6507,7 @@ SecLabelStmt:
 				{
 					SecLabelStmt *n = makeNode(SecLabelStmt);
 					n->provider = $3;
-					n->objtype = OBJECT_TYPE;
+					n->objtype = OBJECT_DOMAIN;
 					n->object = (Node *) $6;
 					n->label = $8;
 					$$ = (Node *) n;