cyassl: fix the test for ASN_NO_SIGNER_E

It's an enum so a macro test won't work. The CyaSSL changelog doesn't
say exactly when this error code was introduced, but it's likely
to be 2.7.0.

lib/vtls/cyassl.c

@@ -300,7 +300,7 @@ cyassl_connect_step2(struct connectdata *conn,
       }
 #endif
     }
-#ifdef ASN_NO_SIGNER_E /* not present in 2.0.6 */
+#if LIBCYASSL_VERSION_HEX >= 0x02007000 /* 2.7.0 */
     else if(ASN_NO_SIGNER_E == detail) {
       if(data->set.ssl.verifypeer) {
         failf(data, "\tCA signer not available for verification\n");