GComboBox: Make set_only_allow_values_from_model() actually take effect

Libraries/LibGUI/GComboBox.cpp

@@ -118,5 +118,6 @@ void GComboBox::set_only_allow_values_from_model(bool b)
 {
     if (m_only_allow_values_from_model == b)
         return;
+    m_only_allow_values_from_model = b;
     m_editor->set_readonly(m_only_allow_values_from_model);
 }