libfetch: do not call deprecated OpenSSL functions

As of OpenSSL 1.1 SSL_library_init() and SSL_load_error_strings() are
deprecated. There are replacement initialization functions but they do
not need to be called: "As of version 1.1.0 OpenSSL will automatically
allocate all resources that it needs so no explicit initialisation is
required."

Wrap both calls in an OPENSSL_VERSION_NUMBER block.

PR: 271615
Reviewed by: Pierre Pronchery <pierre@freebsdfoundation.org>
Event: Kitchener-Waterloo Hackathon 202305
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D40265