MINOR: quic: reject conf with QUIC servers if not compiled

MINOR: quic: reject conf with QUIC servers if not compiled

Ensure that QUIC support is compiled into haproxy when a QUIC server is
configured. This check is performed during _srv_parse_finalize() so that
it is detected both on configuration parsing and when adding a dynamic
server via the CLI.

Note that this changes the behavior of srv_is_quic() utility function.
Previously, it always returned false when QUIC support wasn't compiled.
With this new check introduced, it is now guaranteed that a QUIC server
won't exist if compilation support is not active. Hence srv_is_quic()
does not rely anymore on USE_QUIC define.