using unique_ptr

I have some code that manages SSL and SSL_CTX pointers with unique_ptr.

The following code compiles with OpenSSL 1.0.2, but not with OpenSSL 1.1.1:

std::unique_ptr<SSL>    m_pSession

I include 'openssl/ssl.h', but with OpenSSL 1.1.1 I get the following compile error (using Visual Studio):

error C2027: use of undefined type 'ssl_st'

I have googled a little and it seems that the later version of OpenSSL does not provide the real declaration of ssl_st anywhere? What would be the solution to this?