Add a signal for when a seat updates its capabilities

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
diff --git a/src/compositor.h b/src/compositor.h
index cbeab6f..37ad17f 100644
--- a/src/compositor.h
+++ b/src/compositor.h
@@ -495,6 +495,7 @@
 	struct weston_output *output; /* constraint */
 
 	struct wl_signal destroy_signal;
+	struct wl_signal updated_caps_signal;
 
 	struct weston_compositor *compositor;
 	struct wl_list link;