package/ngircd: add optional support for gnutls

openssl and gnutls can not be enabled at the same time:
https://github.com/ngircd/ngircd/blob/master/configure.ng#L449



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