Fix compiler warning on OS X and Windows.

src/gui/FileDialog.cpp

@@ -67,6 +67,7 @@ QString FileDialog::getSaveFileName(QWidget* parent, const QString& caption, QSt
 
         QString result;
 #if defined(Q_OS_MAC) || defined(Q_OS_WIN)
+        Q_UNUSED(defaultExtension);
         // the native dialogs on these platforms already append the file extension
         result = QFileDialog::getSaveFileName(parent, caption, dir, filter,
                                               selectedFilter, options);