adapt -portable to OpenSSL 1.1x API

Polyfill missing API with replacement functions extracted from LibreSSL
diff --git a/kexgexs.c b/kexgexs.c
index 2a4aa7e..ce934f8 100644
--- a/kexgexs.c
+++ b/kexgexs.c
@@ -36,6 +36,8 @@
 
 #include <openssl/dh.h>
 
+#include "openbsd-compat/openssl-compat.h"
+
 #include "sshkey.h"
 #include "cipher.h"
 #include "digest.h"