Fix unused variable compiler warning in slider example

The example overrides QWidget::resizeEvent() without using the
QResizeEvent * argument. This results in a compiler warning.

This patch marks the argument unused.

Change-Id: I647d0eda7d895e70ed6f232960aec992f5e37b6c
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>
(cherry picked from commit f05aee76288f2cbf021b80ee1b1961bd183cbfbe)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

examples/widgets/widgets/sliders/window.cpp

@@ -116,6 +116,7 @@ void Window::createControls(const QString &title)
 
 void Window::resizeEvent(QResizeEvent *e)
 {
+    Q_UNUSED(e);
     if (width() == 0 || height() == 0)
         return;