ext/openssl/ossl_ssl.c: Use const declaration if LibreSSL >= 2.8.0

to suppress a warning in OpenBSD.

```
ossl_ssl.c:938:31: warning: incompatible pointer types passing 'SSL_SESSION *(SSL *, unsigned char *, int, int *)' (aka 'struct ssl_session_st *(struct ssl_st *, unsigned char *, int, int *)') to parameter of type 'SSL_SESSION *(*)(struct ssl_st *, const unsigned char *, int, int *)' (aka 'struct ssl_session_st *(*)(struct ssl_st *, const unsigned char *, int, int *)') [-Wincompatible-pointer-types]
        SSL_CTX_sess_set_get_cb(ctx, ossl_sslctx_session_get_cb);
                                     ^~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/openssl/ssl.h:738:20: note: passing argument to parameter 'get_session_cb' here
    SSL_SESSION *(*get_session_cb)(struct ssl_st *ssl,
                   ^
1 warning generated.
```

ext/openssl/ossl_ssl.c

@@ -380,7 +380,7 @@ ossl_call_session_get_cb(VALUE ary)
 }
 
 static SSL_SESSION *
-#if OPENSSL_VERSION_NUMBER >= 0x10100000 && !defined(LIBRESSL_VERSION_NUMBER)
+#if (!defined(LIBRESSL_VERSION_NUMBER) ? OPENSSL_VERSION_NUMBER >= 0x10100000 : LIBRESSL_VERSION_NUMBER >= 0x2080000f)
 ossl_sslctx_session_get_cb(SSL *ssl, const unsigned char *buf, int len, int *copy)
 #else
 ossl_sslctx_session_get_cb(SSL *ssl, unsigned char *buf, int len, int *copy)