Fix error on Windows when trying to set set::tls::ecdh-curves.

"[error] ecdh-curves specified but your OpenSSL/LibreSSL library does not
 support setting curves manually by name. Either upgrade to a newer library
 version or remove the 'ecdh-curves' directive from your configuration file"

This also meant the default curves that were offered were up to LibreSSL,
which meant the following list in practice:
 Elliptic curves offered:     prime256v1 secp384r1 secp521r1 X25519 X448
Instead of:
 Elliptic curves offered:     prime256v1 secp384r1 secp521r1 X25519

Not that X448 is considered bad, it just didn't match what we claimed in
the docs at https://www.unrealircd.org/docs/TLS_Ciphers_and_protocols

Fixed by: #define HAS_SSL_CTX_SET1_CURVES_LIST

include/windows/setup.h

@@ -41,6 +41,7 @@
 #define HAVE_EXPLICIT_BZERO
 #define HAVE_STRNLEN
 #define explicit_bzero(a,b) SecureZeroMemory(a,b)
+#define HAS_SSL_CTX_SET1_CURVES_LIST
 
 /* mode_t: Needed in s_conf.c for the third argument of open(3p).
  * Should be an int because of http://msdn.microsoft.com/en-us/library/z0kc8e3z(VS.71).aspx