bio/bss_dgram.c: harmonize usage of OPENSSL_USE_IPV6 with the rest.

Reviewed-by: NRich Salz <rsalz@openssl.org>
Reviewed-by: NKurt Roeckx <kurt@roeckx.be>
(Merged from https://github.com/openssl/openssl/pull/6745)

crypto/bio/bss_dgram.c

@@ -364,7 +364,7 @@ static long dgram_get_mtu_overhead(bio_dgram_data *data)
          */
         ret = 28;
         break;
-# ifdef AF_INET6
+# if OPENSSL_USE_IPV6
     case AF_INET6:
         {
 #  ifdef IN6_IS_ADDR_V4MAPPED