www/elinks: Fix build on OpenSSL 1.1.1

[cem]

Summary

Use the provided getter routine instead of directly accessing the contents of
the SSL struct, which is no longer exported as part of the ABI.

Test Plan

It builds and seems to run fine. I don't have any sites that actually use
client certificates to test that particular feature.

[beat]

Thank you!

[cem]

No problem!