ppp: specify OpenSSL 1.1 APIs

OPENSSL_API_COMPAT can be used to specify the OpenSSL API version in
use for the purpose of hiding deprecated interfaces and enabling
the appropriate deprecation notices.

This change is a NFC while we're still using OpenSSL 1.1.1 but will
avoid deprecation warnings upon the switch to OpenSSL 3.0.

A future change should migrate to use the OpenSSL 3.0 APIs.

PR: 271615
Reviewed by: emaste
Event: Kitchener-Waterloo Hackathon 202305
Sponsored by: The FreeBSD Foundation