adapt -portable to OpenSSL 1.1x API

Polyfill missing API with replacement functions extracted from LibreSSL
diff --git a/kexdhs.c b/kexdhs.c
index 8367c6c..337aab5 100644
--- a/kexdhs.c
+++ b/kexdhs.c
@@ -35,6 +35,8 @@
 
 #include <openssl/dh.h>
 
+#include "openbsd-compat/openssl-compat.h"
+
 #include "sshkey.h"
 #include "cipher.h"
 #include "digest.h"