Fix compiler warning on rd_kafka_TopicPartitionInfo_partition

[dzbarsky]

The const qualifier is pointless and results in compilation warnings on Clang

[confluent-cla-assistant]

🎉 All Contributor License Agreements have been signed. Ready to merge.
✅ dzbarsky
_{Please push an empty commit if you would like to re-run the checks to verify CLA status for all contributors.}