Fix compilation with --with-gssapi --without-openssl

The #define is spelled ENABLE_GSS, not USE_GSS. Introduced in commit
05fd30c, reported by Thomas Munro.

Discussion: https://www.postgresql.org/message-id/CA%2BhUKG%2BHRTtB%2Bx%2BKKKj_cfX6sNhbeGuqmGxjGMwdVPG7YGFP8w@mail.gmail.com

Author: hlinnaka

src/interfaces/libpq/fe-connect.c

@@ -393,7 +393,7 @@ static const char short_uri_designator[] = "postgres://";
 
 static bool connectOptions1(PGconn *conn, const char *conninfo);
 static bool init_allowed_encryption_methods(PGconn *conn);
-#if defined(USE_SSL) || defined(USE_GSS)
+#if defined(USE_SSL) || defined(ENABLE_GSS)
 static int	encryption_negotiation_failed(PGconn *conn);
 #endif
 static bool connection_failed(PGconn *conn);
@@ -4318,7 +4318,7 @@ init_allowed_encryption_methods(PGconn *conn)
  *
  * conn->current_enc_method is updated to the next method to try.
  */
-#if defined(USE_SSL) || defined(USE_GSS)
+#if defined(USE_SSL) || defined(ENABLE_GSS)
 static int
 encryption_negotiation_failed(PGconn *conn)
 {