/usr/include/postgresql/ is in libpq-dev 9.5.14-0ubuntu0.16.04.
This file is owned by root:root, with mode 0o755.
..
/usr/include/postgresql/internal/
/usr/include/postgresql/internal/c.h
/usr/include/postgresql/internal/libpq/
/usr/include/postgresql/internal/libpq-int.h
/usr/include/postgresql/internal/libpq/pqcomm.h
/usr/include/postgresql/internal/port.h
/usr/include/postgresql/internal/postgres_fe.h
/usr/include/postgresql/internal/pqexpbuffer.h
/usr/include/postgresql/libpq/
/usr/include/postgresql/libpq-events.h
/usr/include/postgresql/libpq-fe.h
/usr/include/postgresql/libpq/libpq-fs.h
/usr/include/postgresql/pg_config.h
/usr/include/postgresql/pg_config_ext.h
/usr/include/postgresql/pg_config_manual.h
/usr/include/postgresql/pg_config_os.h
/usr/include/postgresql/postgres_ext.h