package/ngircd: make zlib an optional dependency

zlib is not a hard dependency:
https://github.com/ngircd/ngircd/blob/master/configure.ng#L256



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>