kafka: fix log callback signature

The first argument is expected to be constant. This has always been the
case for `rd_kafka_conf_set_log_cb()` function (see
https://github.com/edenhill/librdkafka/commit/a0d4400fe9032532a34f3d97ab6f0b70bf7d28c0)
and it seems that it has always been the case for
`rd_kafka_set_logger()` too (see
https://github.com/edenhill/librdkafka/commit/96b461570a74ad83c729172e27b72543d74d0ffd).