postfix: add link file /usr/lib/sendmail

Link file /usr/lib/sendmail points to /usr/sbin/sendmail is required by
LSB core test according to Linux FHS[Ref2]: "For historical reasons,
/usr/lib/sendmail must be a symbolic link to /usr/sbin/sendmail if the
latter exists."

The linke file was provided by package lsb before, but should be
provided by packages which provides command sendmail such as msmtp,
postfix and esmtp etc.

Refs:
1 http://refspecs.linuxfoundation.org/LSB_4.1.0/LSB-Core-generic/LSB-Core-generic/normativerefs.html#STD.
2 http://www.pathname.com/fhs/pub/fhs-2.3.html#SPECIFICOPTIONS13



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>