postfix: fix bug of main.cf_2.0

${exec_prefix}/libexec/${PN} => ${libexecdir}

Signed-off-by: Bian Naimeng <biannm@cn.fujitsu.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Joe MacDonald <joe_macdonald@mentor.com>