package/exim: security bump to version 4.89.1

Fixes CVE-2017-16943 & CVE-2017-16944:
https://lists.exim.org/lurker/message/20171128.215505.79ea8efa.en.html



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>