linux-dmabuf-v1: fix missing SCANOUT flag

options->scanout_primary_output indicates an output on which scanout is desirable.
Set the missing SCANOUT flag.

Fixes: 0c966f102c ("linux-dmabuf-v1: add wlr_linux_dmabuf_feedback_v1_init_with_options()")

types/wlr_linux_dmabuf_v1.c

@@ -1109,6 +1109,7 @@ bool wlr_linux_dmabuf_feedback_v1_init_with_options(struct wlr_linux_dmabuf_feed
 		}
 
 		tranche->target_device = backend_dev;
+		tranche->flags = ZWP_LINUX_DMABUF_FEEDBACK_V1_TRANCHE_FLAGS_SCANOUT;
 		if (!wlr_drm_format_set_intersect(&tranche->formats, scanout_formats, renderer_formats)) {
 			wlr_log(WLR_ERROR, "Failed to intersect renderer and scanout formats");
 			goto error;