[gupnp] Update gupnp-service-info.h, add nullable anotation,

[Jens Georg <jensgeorg src gnome org>]

commit b01ce911ecdf6d0f75c71153903728f9dde09d7e
Author: soiamsoNG <83182235 qq com>
Date:   Wed Jun 10 11:46:54 2020 +0000

    Update gupnp-service-info.h, add nullable anotation,

 libgupnp/gupnp-service-info.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/libgupnp/gupnp-service-info.h b/libgupnp/gupnp-service-info.h
index e700979..2d7360f 100644
--- a/libgupnp/gupnp-service-info.h
+++ b/libgupnp/gupnp-service-info.h
@@ -52,8 +52,8 @@ struct _GUPnPServiceInfoClass {
 /**
  * GUPnPServiceIntrospectionCallback:
  * @info: The #GUPnPServiceInfo introspection was requested for
- * @introspection: The new #GUPnPServiceIntrospection object, or NULL
- * @error: The #GError that occurred, or NULL
+ * @introspection: (nullable): The new #GUPnPServiceIntrospection object, or NULL
+ * @error: (nullable): The #GError that occurred, or NULL
  * @user_data: User data
  *
  * Callback notifying that @introspection for @info has been obtained.