timesyncd: generate a structure log message the first time we set the clock correctly

timesyncd: generate a structure log message the first time we set the clock correctly

Usecase: later on we can use this to retroactively adjust log output in
journalctl or similar on systems lacking an RTC: we just have to search
for this sructured log message that indicates the first sync point and
can then retroactively adjust the incorrect timestamps collected before
that.