X11: Fix creating RenderingDevice after godotengine#93706

The line was removed by mistake.

platform/linuxbsd/x11/display_server_x11.cpp

@@ -6261,6 +6261,7 @@ DisplayServerX11::DisplayServerX11(const String &p_rendering_driver, WindowMode
 
 #if defined(RD_ENABLED)
 	if (rendering_context) {
+		rendering_device = memnew(RenderingDevice);
 		if (rendering_device->initialize(rendering_context, MAIN_WINDOW_ID) != OK) {
 			memdelete(rendering_device);
 			rendering_device = nullptr;