Fix some compiler warnings

Change-Id: I67135a49c7bc8169dd159cdab381c21f27be02f3
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/qmldesigner/designersettings.cpp

@@ -34,6 +34,7 @@ void DesignerSettings::insert(const QHash<QByteArray, QVariant> &settingsHash)
 
 QVariant DesignerSettings::value(const QByteArray &key, const QVariant &defaultValue) const
 {
+    Q_UNUSED(defaultValue)
     QMutexLocker locker(&m_mutex);
     return m_cache.value(key);
 }