Change debug log level to warn in in_syslog

Pattern mismatch log should be warn for early detection.
We can disable this message by `log_level error` in production.

https://groups.google.com/d/msg/fluentd/ZRreaaRFR2U/ineHN8FOGI0J
Hmmm. Should it be `error` level?
The error occurred in `emit` so it seems to no warning level.