Add missing config schema

Hi,

I have noticed this module does not provide a config schema for the third-party settings it manipulates.

You can notice this by doing the following steps:

1. Create a fresh, standard Drupal site
2. Enable module menu_force
3. Go to admin/structure/types/manage/page and, under tab Menu settings, check Make the Menu Settings mandatory for this content type (which is provided by this module) and then save
4. Enable module config_inspector
5. Go to admin/reports/config-inspector: you will notice 2 schema errors on node.type.page (see screenshot menu_force_config_schema_error_overview.png)
6. Go to admin/reports/config-inspector/node.type.page/list: you will notice that the schema errors are caused by menu_force (see screenshot menu_force_config_schema_error_details.png)

Any chance this can be added ? Attached is a patch that solves the problem for us.