timedate: increment reference count of sd_bus_message

The commit 5d280742 introduces a
barrier to suppress calling context_update_ntp_status() multiple times.
However, it just stores the address of sd_bus_message object. So,
when an address is reused on the subsequent message, then the status
of NTP clients are not updated.

This makes the stored message object is referenced by the context
object. So, the subsequent message is on cirtainly different address.