net-p2p/litecoin: ignore when SSL_DEFAULT matches "libressl"

If an attempt to build with LibreSSL is made, the configure script emits
this message: configure: error: Detected LibreSSL:

This is not supported, and may break consensus compatibility

Go ahead and mark this port and its slaved with IGNORE when libressl is
used to save the user the trouble of hitting the error later (this is
also done by security/p5-openxpki)

Approved by: SSL blanket