GenericProjectManager: Pass string by reference

Change-Id: I4eb8ecbf477764b780c1ef76c179af550e3952d3
Reviewed-by: Daniel Teske <daniel.teske@digia.com>

src/plugins/genericprojectmanager/genericprojectnodes.cpp

@@ -57,7 +57,7 @@ QString GenericProjectNode::projectFilePath() const
     return m_projectFile->filePath();
 }
 
-QHash<QString, QStringList> sortFilesIntoPaths(const QString &base, const QSet<QString> files)
+QHash<QString, QStringList> sortFilesIntoPaths(const QString &base, const QSet<QString> &files)
 {
     QHash<QString, QStringList> filesInPath;
     const QDir baseDir(base);