package/syslog-ng: add optional support for libuuid from util-linux

When libuuid was compiled before, syslog-ng will use it as optional
dependency:

$ output/host/usr/bin/x86_64-linux-readelf -a output/target/usr/lib/libsyslog-ng-3.7.so.0.0.0 | grep NEEDED | sort
[...]
 0x0000000000000001 (NEEDED)             Shared library: [libuuid.so.1]

The build system offers no option to en-/disable libuuid support:
https://github.com/balabit/syslog-ng/blob/master/configure.ac#L1291



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