syslog-ng: Add upstream patches to support python 3

>From https://github.com/balabit/syslog-ng/pull/711

Fixes:
http://autobuild.buildroot.net/results/8bf/8bfb199bda2218252bc21b228066d41af37e2753/
http://autobuild.buildroot.net/results/ab8/ab8ec583d1836d0c246ac792391549bfe277ce76/



Signed-off-by: Chris Packham <judge.packham@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>