znc: HEAD builds depend on OpenSSL

Recent ZNC master fails to build on Mavericks, because Csocket started using TLS 1.2 support in OpenSSL.

Csocket commit introducing TLS 1.2 support: https://github.com/jimloco/Csocket/commit/a1aa92060d314be247b75d41d1f68c6957daaa3f
ZNC commit importing Csocket changes: https://github.com/znc/znc/commit/5e90bc380bae9e2278ffe0b0ba0983bb0789c05f