session: Add missing ifdef that prevented Windows builds

Signed-off-by: Tilo Eckert <tilo.eckert@flam.de>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>
(cherry picked from commit 4f392ebc7e)

src/session.c

@@ -270,7 +270,9 @@ void ssh_free(ssh_session session) {
       ssh_list_free(session->opts.identity);
   }
 
+#ifndef _WIN32
   ssh_agent_state_free (session->agent_state);
+#endif
   session->agent_state = NULL;
 
   SAFE_FREE(session->auth_auto_state);