/usr/include/postgresql/9.6/server/distributed/ is in postgresql-9.6-citus 6.0.1.PGDG-1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/postgresql/9.6/server/distributed/citus_clauses.h
/usr/include/postgresql/9.6/server/distributed/citus_nodefuncs.h
/usr/include/postgresql/9.6/server/distributed/citus_nodes.h
/usr/include/postgresql/9.6/server/distributed/citus_ruleutils.h
/usr/include/postgresql/9.6/server/distributed/colocation_utils.h
/usr/include/postgresql/9.6/server/distributed/commit_protocol.h
/usr/include/postgresql/9.6/server/distributed/connection_cache.h
/usr/include/postgresql/9.6/server/distributed/distribution_column.h
/usr/include/postgresql/9.6/server/distributed/listutils.h
/usr/include/postgresql/9.6/server/distributed/master_metadata_utility.h
/usr/include/postgresql/9.6/server/distributed/master_protocol.h
/usr/include/postgresql/9.6/server/distributed/metadata_cache.h
/usr/include/postgresql/9.6/server/distributed/metadata_sync.h
/usr/include/postgresql/9.6/server/distributed/multi_client_executor.h
/usr/include/postgresql/9.6/server/distributed/multi_copy.h
/usr/include/postgresql/9.6/server/distributed/multi_executor.h
/usr/include/postgresql/9.6/server/distributed/multi_explain.h
/usr/include/postgresql/9.6/server/distributed/multi_join_order.h
/usr/include/postgresql/9.6/server/distributed/multi_logical_optimizer.h
/usr/include/postgresql/9.6/server/distributed/multi_logical_planner.h
/usr/include/postgresql/9.6/server/distributed/multi_master_planner.h
/usr/include/postgresql/9.6/server/distributed/multi_physical_planner.h
/usr/include/postgresql/9.6/server/distributed/multi_planner.h
/usr/include/postgresql/9.6/server/distributed/multi_resowner.h
/usr/include/postgresql/9.6/server/distributed/multi_router_executor.h
/usr/include/postgresql/9.6/server/distributed/multi_router_planner.h
/usr/include/postgresql/9.6/server/distributed/multi_server_executor.h
/usr/include/postgresql/9.6/server/distributed/multi_shard_transaction.h
/usr/include/postgresql/9.6/server/distributed/multi_utility.h
/usr/include/postgresql/9.6/server/distributed/pg_dist_colocation.h
/usr/include/postgresql/9.6/server/distributed/pg_dist_local_group.h
/usr/include/postgresql/9.6/server/distributed/pg_dist_node.h
/usr/include/postgresql/9.6/server/distributed/pg_dist_partition.h
/usr/include/postgresql/9.6/server/distributed/pg_dist_shard.h
/usr/include/postgresql/9.6/server/distributed/pg_dist_shard_placement.h
/usr/include/postgresql/9.6/server/distributed/pg_dist_transaction.h
/usr/include/postgresql/9.6/server/distributed/relay_utility.h
/usr/include/postgresql/9.6/server/distributed/resource_lock.h
/usr/include/postgresql/9.6/server/distributed/shardinterval_utils.h
/usr/include/postgresql/9.6/server/distributed/task_tracker.h
/usr/include/postgresql/9.6/server/distributed/task_tracker_protocol.h
/usr/include/postgresql/9.6/server/distributed/test_helper_functions.h
/usr/include/postgresql/9.6/server/distributed/transaction_recovery.h
/usr/include/postgresql/9.6/server/distributed/transmit.h
/usr/include/postgresql/9.6/server/distributed/worker_manager.h
/usr/include/postgresql/9.6/server/distributed/worker_protocol.h
/usr/include/postgresql/9.6/server/distributed/worker_transaction.h