/usr/include/mysql/mysql/ is in libmysqlclient-dev 5.5.54-0ubuntu0.12.04.1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/mysql/mysql/client_plugin.h
/usr/include/mysql/mysql/innodb_priv.h
/usr/include/mysql/mysql/plugin.h
/usr/include/mysql/mysql/plugin_audit.h
/usr/include/mysql/mysql/plugin_auth.h
/usr/include/mysql/mysql/plugin_auth_common.h
/usr/include/mysql/mysql/plugin_ftparser.h
/usr/include/mysql/mysql/psi/
/usr/include/mysql/mysql/psi/mysql_file.h
/usr/include/mysql/mysql/psi/mysql_thread.h
/usr/include/mysql/mysql/psi/psi.h
/usr/include/mysql/mysql/psi/psi_abi_v1.h
/usr/include/mysql/mysql/psi/psi_abi_v2.h
/usr/include/mysql/mysql/service_my_snprintf.h
/usr/include/mysql/mysql/service_thd_alloc.h
/usr/include/mysql/mysql/service_thd_wait.h
/usr/include/mysql/mysql/service_thread_scheduler.h
/usr/include/mysql/mysql/services.h
/usr/include/mysql/mysql/thread_pool_priv.h