syslog-ng: fix build with newer json-c

Add upstream patch fixing build with json-c version 0.13.

Fixes:
http://autobuild.buildroot.net/results/e12/e12b88d082f053247863124f4802cafc73278c8c/
http://autobuild.buildroot.net/results/cdc/cdc941b47fe04e8e99393004b750451490dc8c8e/
http://autobuild.buildroot.net/results/b85/b854c97b5af5125394246298d010baafa175a4c3/



Cc: Chris Packham <judge.packham@gmail.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>