message: Ubuntu 18 compile fix
diff: #include <openssl/ecdsa.h>
  #include <openssl/rand.h>
  #include <openssl/obj_mac.h>
  +#include <openssl/opensslv.h>
  +
  +#if OPENSSL_VERSION_NUMBER >= 0x10100000L
  +typedef struct ECDSA_SIG_st {
  + BIGNUM *r;
  + BIGNUM *s;
  +} ECDSA_SIG;
  +#endif
  static secp256k1_context* secp256k1_context_sign = nullptr;
  