BUG/MINOR: ssl: SSL_load_error_strings might not be defined

BUG/MINOR: ssl: SSL_load_error_strings might not be defined

The SSL_load_error_strings function was marked as deprecated in OpenSSL
1.1.0 so compiling HAProxy with OPENSSL_NO_DEPRECATED set and a recent
OpenSSL library would fail.
The manpages say that this function was replaced by OPENSSL_init_crypto
and OPENSSL_init_ssl which are already called at start up by the SSL
lib. We do not seem to be in a case where explicit call of those
functions is required.

This patch fixes GitHub issue #1813.
It can be backported to 2.6.