C++: Fix crash when deducing auto (with invalid ids)

Task-number: QTCREATORBUG-7801

Change-Id: I61436148ed1428a50a6840ba9109310a57dabaa4
Reviewed-by: Roberto Raggi <roberto.raggi@nokia.com>

src/libs/cplusplus/LookupContext.h

@@ -252,6 +252,7 @@ private:
     QSharedPointer<Control> _control;
 };
 
+bool CPLUSPLUS_EXPORT compareName(const Name *name, const Name *other);
 bool CPLUSPLUS_EXPORT compareFullyQualifiedName(const QList<const Name *> &path,
                                                 const QList<const Name *> &other);