Fix a missing const

src/gui_elements/Datepicker.cc

@@ -103,7 +103,7 @@ Datepicker::~Datepicker()
  */
 void Datepicker::setOptions(const QDomNode &options)
 {
-	QString user_date_format = options.toElement().attribute("format");
+	const QString user_date_format = options.toElement().attribute("format");
 	if (!user_date_format.isEmpty())
 		date_format_ = user_date_format;
 	datepicker_->setDisplayFormat(date_format_);