postfix: update daemon_directory in main.cf_2.0

daemon_directory is set to /usr/lib/postfix which causes daemon postfix
fails to start on 64 bits target if enable multilib. Set daemon_directory
with libexecdir to fix it.

Signed-off-by: Kai Kang <kai.kang@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Joe MacDonald <joe_macdonald@mentor.com>