package/lxc: switch optional gnutls dependency to openssl

lxc switched from gnutls to openssl since version 3.2.0:
https://github.com/lxc/lxc/commit/fa2bb6ba532c5e7f92df8cbae50a68af519f9997

Patch was originally created by Fabrice Fontaine:
http://patchwork.ozlabs.org/patch/1148360/



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>