msmtp: 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: "For historical reasons,
/usr/lib/sendmail must be a symbolic link to /usr/sbin/sendmail if the
latter exists."

Create the link file /usr/lib/sendmail if it doesn't exist.

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



Signed-off-by: Kai Kang <kai.kang@windriver.com>