x11_standalone_overlaywindow: Adjust initialization order

This fixes a clang warning about fields being initialized in a different order than they were declared

src/backends/x11/standalone/x11_standalone_overlaywindow.cpp

@@ -30,9 +30,9 @@ namespace KWin
 OverlayWindowX11::OverlayWindowX11(X11StandaloneBackend *backend)
     : OverlayWindow()
     , X11EventFilter(QList<int>{XCB_EXPOSE, XCB_VISIBILITY_NOTIFY})
-    , m_backend(backend)
     , m_visible(true)
     , m_shown(false)
+    , m_backend(backend)
     , m_window(XCB_WINDOW_NONE)
 {
 }