For the case that the document is based upon another one (see
Snapshot::preprocessedDocument(), the only client of FastPreprocessor)
the FastPreprocessor added extra unresolved includes.

Change-Id: I0a7719fa8806af59f87a48bc6914270efd653e84
Reviewed-by: Erik Verbruggen <erik.verbruggen@theqtcompany.com>