Rename wl_data_offer to weston_data_offer

Another left-over from when we moved the input structs.
diff --git a/src/compositor.h b/src/compositor.h
index a045839..0fca43d 100644
--- a/src/compositor.h
+++ b/src/compositor.h
@@ -275,7 +275,7 @@
 	struct weston_touch *touch;
 };
 
-struct wl_data_offer {
+struct weston_data_offer {
 	struct wl_resource *resource;
 	struct weston_data_source *source;
 	struct wl_listener source_destroy_listener;