postfix: Fix rootfs creation errors

1) error: file /usr/share/man/man1/mailq.1 from install of postfix-doc
        conflicts with file from package esmtp-doc
2) error: file /usr/share/man/man1/newaliases.1 from install of postfix-doc
        conflicts with file from package esmtp-doc
3) error: file /usr/share/man/man1/sendmail.1 from install of postfix-doc
        conflicts with file from package esmtp-doc

Signed-off-by: Dai Caiyun <daicy.fnst@cn.fujitsu.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Joe MacDonald <joe_macdonald@mentor.com>
(cherry picked from commit 7832c673504415cf78a910263c23dda8ec9b982d)
Signed-off-by: Armin Kuster <akuster808@gmail.com>