sd-netlink: make timeout message sealed

When sd_netlink_call_async() timed out, then we reply the synthetic
error message, but it was not sealed. So, reading the message causes
the following assertion:

```
Assertion 'm->sealed' failed at src/libsystemd/sd-netlink/netlink-message.c:652, function netlink_message_read_internal(). Ignoring.
```