Fix sources for modern ssh

[hroff-1902]

...to allow the app to be compiled under actual linuxes.

Resolves #508

[satoshi75nakamoto]

I can confirm that this fixes:

In file included from /Users/poreilly/code/blackbird/src/exchanges/exmo.cpp:7:
/Users/poreilly/code/blackbird/src/utils/hmac_sha512.hpp:11:12: error: variable has incomplete type 'HMAC_CTX' (aka 'hmac_ctx_st')
                HMAC_CTX ctx;
                         ^
/usr/local/Cellar/[email protected]/1.1.1h/include/openssl/ossl_typ.h:102:16: note: forward declaration of 'hmac_ctx_st'
typedef struct hmac_ctx_st HMAC_CTX;
               ^
1 error generated.