LibJS: Suppress LibJSGCVerifier warning about Map::m_keys

Userland/Libraries/LibJS/Runtime/Map.cpp

@@ -86,6 +86,8 @@ void Map::visit_edges(Cell::Visitor& visitor)
         visitor.visit(value.key);
         visitor.visit(value.value);
     }
+    // NOTE: The entries in m_keys are already visited by the walk over m_entries above.
+    visitor.ignore(m_keys);
 }
 
 }