xo-reader2: + SyntaxStateMachine.forward_children method

Support gc traversal with goal of making ParserStateMachine a gc root

src/reader2/DExpectQLiteralSsm.cpp

@@ -255,6 +255,12 @@ namespace xo {
                                              "DExpectQLiteralSsm",
                                              refrtag("expect", this->get_expect_str()));
         }
+        void
+        DExpectQLiteralSsm::forward_children(obj<ACollector> /*gc*/) noexcept
+        {
+            // cxl_on_rightparen_, cxl_on_rightbracket_: POD, skip
+        }
+
     } /*namespace scm*/
 } /*namespace xo*/