egl/x11/dri3: adding missing __DRI_BACKGROUND_CALLABLE extension

Fixes: 3b7b6adf3a ("egl: Implement __DRI_BACKGROUND_CALLABLE")
Cc: Timothy Arceri <tarceri@itsqueeze.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>

src/egl/drivers/dri2/platform_x11.c

@@ -1289,6 +1289,7 @@ static const __DRIextension *dri3_image_loader_extensions[] = {
    &dri3_image_loader_extension.base,
    &image_lookup_extension.base,
    &use_invalidate.base,
+   &background_callable_extension.base,
    NULL,
 };