BUG/MINOR: quic: Wrong keylog callback setting.

BUG/MINOR: quic: Wrong keylog callback setting.

This bug impacts only the QUIC OpenSSL compatibility module (USE_QUIC_OPENSSL_COMPAT).

To make this module works, quic_tls_compat_keylog_callback() function must be
set as keylog callback, or at least be called by another keylog callback.
This is what SSL_CTX_keylog() was supposed to do. In addition to export the TLS
secrets via sample fetches this latter also calls quic_tls_compat_keylog_callback()
when compiled with USE_QUIC_OPENSSL_COMPAT defined.

Before this patch, SSL_CTX_keylog() was replaced by quic_tls_compat_keylog_callback()
and the TLS secret were no more exported by sample fetches.

Must be backported to 2.8.