package/syslog-ng: add optional support for geoip

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

$ output/host/usr/bin/x86_64-linux-readelf -a output/target/usr/lib/syslog-ng/libgeoip-plugin.so | grep NEEDED | sort
 0x0000000000000001 (NEEDED)             Shared library: [ld64-uClibc.so.1]
 0x0000000000000001 (NEEDED)             Shared library: [libc.so.1]
 0x0000000000000001 (NEEDED)             Shared library: [libGeoIP.so.1]
 0x0000000000000001 (NEEDED)             Shared library: [libpthread.so.1]
 0x0000000000000001 (NEEDED)             Shared library: [librt.so.1]
 0x0000000000000001 (NEEDED)             Shared library: [libsyslog-ng-3.7.so.0]

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