C++: FastPreprocessor: Add only includes if there are none

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>

src/libs/cplusplus/FastPreprocessor.h

@@ -49,6 +49,7 @@ class CPLUSPLUS_EXPORT FastPreprocessor: public Client
     Preprocessor _preproc;
     QSet<QString> _merged;
     Document::Ptr _currentDoc;
+    bool _addIncludesToCurrentDoc;
 
     void mergeEnvironment(const QString &fileName);