mkosi: Fix mkosi.clangd (#36387)

- Add missing '--' delimiter
- Use the new BuildSubdirectory JSON field to figure out the build
  subdirectory.
- Remove the /usr/include path mapping for now. This means we can't
  jump into system headers anymore if they don't exist on the host,
  we can find a way to add this back later if it turns out to be
  crucial.