message: define to make cran happier
diff: #include "mbedtls_wrapper.hpp"
  +// otherwise we have different definitions for mbedtls_pk_context / mbedtls_sha256_context
  +#define MBEDTLS_ALLOW_PRIVATE_ACCESS
  #include "mbedtls/sha256.h"
  #include "mbedtls/pk.h"
  