Fix failure when building webkit without wayland support

--- tdesktop-5.8.3-full.orig/Telegram/lib_webview/webview/platform/linux/webview_linux_webkitgtk.cpp
+++ tdesktop-5.8.3-full/Telegram/lib_webview/webview/platform/linux/webview_linux_webkitgtk.cpp
@@ -730,9 +730,11 @@
 
 void Instance::setOpaqueBg(QColor opaqueBg) {
 	if (_remoting) {
+#ifdef DESKTOP_APP_WEBVIEW_WAYLAND_COMPOSITOR
 		if (const auto widget = qobject_cast<QQuickWidget*>(_widget.get())) {
 			widget->setClearColor(opaqueBg);
 		}
+#endif
 
 		if (!_helper) {
 			return;
