syslog-ng: Link against librt

Some of the rabbit-mq tools need to link against librt, specify
LIBS=-lrt when configuring.

Fixes:
http://autobuild.buildroot.net/results/a3d/a3dfdebd76d47df11a8e26a215bb88fbce8dc6f7
http://autobuild.buildroot.net/results/7d2/7d26e8efe67bf958162a309acb20800e669543e4
http://autobuild.buildroot.net/results/7b5/7b5370d12a2ff8937cba3a201d4edce3dcd12a6e
http://autobuild.buildroot.net/results/bab/babbb089ce1a7750495323248f21402960fb6c80



Helped-by: Waldemar Brodkorb <wbx@openadk.org>
Signed-off-by: Chris Packham <judge.packham@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>