Add Q_UNUSED to avoid compilation errors

Variable unused causes compile error or warning, use Q_UNUSED to eliminate it.

Change-Id: I6efbc3150cef962fa45ccbbed0dc04794db3cbbe
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@qt.io>

src/plugins/platformthemes/gtk3/qgtk3theme.cpp

@@ -254,6 +254,7 @@ const QFont *QGtk3Theme::font(Font type) const
 QIcon QGtk3Theme::fileIcon(const QFileInfo &fileInfo,
                            QPlatformTheme::IconOptions iconOptions) const
 {
+    Q_UNUSED(iconOptions);
     Q_ASSERT(m_storage);
     return m_storage->fileIcon(fileInfo);
 }