CMake: Avoid a bit of useless processing when updating the project tree

Do not pass around a list of filenodes with all the known header files.
That list gets converted into a QSet<FilePath> and then that is used.

Just generate the QSet<FilePath> directly and avoid that conversion.

Change-Id: I2444a2a6b4a1600fe476e66673a1a2e9c8900764
Reviewed-by: Cristian Adam <cristian.adam@qt.io>
Reviewed-by: hjk <hjk@qt.io>

src/plugins/cmakeprojectmanager/projecttreehelper.h

@@ -67,7 +67,7 @@ CMakeTargetNode *createTargetNode(
     const QString &displayName);
 
 void addHeaderNodes(ProjectExplorer::ProjectNode *root,
-                    const QVector<ProjectExplorer::FileNode *> knownHeaders,
+                    QSet<Utils::FilePath> &seenHeaders,
                     const QList<const ProjectExplorer::FileNode *> &allFiles);
 
 } // namespace Internal