move activity_notify into events in wlr_idle

This moves the `struct wl_signal activity_notify` in `struct wlr_idle`
into a local `struct {} events` to keep consistency with other modules
in the library.

include/wlr/types/wlr_idle.h

@@ -19,7 +19,9 @@ struct wlr_idle {
 	struct wl_event_loop *event_loop;
 
 	struct wl_listener display_destroy;
-	struct wl_signal activity_notify;
+	struct {
+		struct wl_signal activity_notify;
+	} events;
 
 	void *data;
 };