BUG/MINOR: proxy: fix default ALPN bind settings

BUG/MINOR: proxy: fix default ALPN bind settings

For "add backend" implementation, postparsing code in
check_config_validity() from cfgparse.c has been extracted in a new
dedicated function named proxy_finalize() into proxy.c.

This has caused unexpected compilation issue as in the latter file
TLSEXT_TYPE_application_layer_protocol_negotiation macro may be
undefined, in particular when building without QUIC support. Thus, code
related to default ALPN on binds is discarded after the preprocessing
stage.

Fix this by including openssl-compat header file into proxy source file.
This should be sufficient to ensure SSL related defines are properly
included.

This should fix recent issues on SSL regtests.

No need to backport.