dtl_mtu_test doesn't follow BIO_* conventions and make Windows build fail

Reviewed-by: NKurt Roeckx <kurt@openssl.org>
Reviewed-by: NRichard Levitte <levitte@openssl.org>

test/dtls_mtu_test.c

@@ -26,7 +26,7 @@ static unsigned int clnt_psk_callback(SSL *ssl, const char *hint,
                                       unsigned char *psk,
                                       unsigned int max_psk_len)
 {
-    snprintf(ident, max_ident_len, "psk");
+    BIO_snprintf(ident, max_ident_len, "psk");
 
     if (max_psk_len > 20)
         max_psk_len = 20;