Fix Buffer Serialize struct length

[jeremy-prater]

I noticed that rmp-serde and serde_cbor/ciborium both fail to deserialize the gstreamer::Buffer object because the encoded struct size is incorrect. There are 7 elements in the struct, but the serializer only specifies 6.

It is curious that serde_json worked correctly, but binary serialization did not.

[sdroege]

Thanks. This is only a mirror of the official repository. Can you create a MR here please: https://gitlab.freedesktop.org/gstreamer/gstreamer-rs ?