rpm centos: specify correct directory for libmysqlservices

mroonga is linked with MySQL 5.6.x on CentOS 5.
MySQL 5.6.x requires libmysqlservices and
libmysqlservices is installed under /usr/lib64/mysql, not /usr/lib64.

It is better to use `mysql_config --plugindir` because it returns
/usr/lib64/mysql/plugin.