gsettings: add the modules schema to the schema description file

gsettings: add the modules schema to the schema description file

Originally the idea was to provide the "modules" schema with paprefs,
but since module-gsettings refers to the "modules" schema in its code,
that would make module-gsettings depend on paprefs, which is not good.
Now all schemas are provided by module-gsettings, so the paprefs
dependency is avoided. Unfortunately this means that if paprefs is
modified to load some new modules, the schema in pulseaudio needs to be
updated as well.