compositor: Track pointer image protocol change

The request pointer.attach was replaced with pointer.set_cursor that
lets a client set the cursor surface for a given pointer.
diff --git a/src/compositor-wayland.c b/src/compositor-wayland.c
index f09463e..d498e84 100644
--- a/src/compositor-wayland.c
+++ b/src/compositor-wayland.c
@@ -505,7 +505,7 @@
 
 	output = wl_surface_get_user_data(surface);
 	notify_pointer_focus(&c->base.seat->seat, &output->base, x, y);
-	wl_pointer_attach(input->pointer, serial, NULL, 0, 0);
+	wl_pointer_set_cursor(input->pointer, serial, NULL, 0, 0);
 }
 
 static void