QmlDesigner: avoid unused warning in release

Change-Id: I6213fd3555b28b14d23453c24b94acef5baeaf3a
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@theqtcompany.com>

src/plugins/qmldesigner/designmodewidget.cpp

@@ -317,6 +317,7 @@ void DesignModeWidget::setup()
     m_warningWidget->setVisible(false);
     connect(m_warningWidget.data(), &DocumentWarningWidget::gotoCodeClicked, [=]
         (const QString &filePath, int codeLine, int codeColumn) {
+            Q_UNUSED(filePath);
             Q_ASSERT(textEditor()->textDocument()->filePath().toString() == filePath);
             textEditor()->gotoLine(codeLine, codeColumn);
             Core::ModeManager::activateMode(Core::Constants::MODE_EDIT);