Add sshchat dependencies in vendor

vendor/golang.org/x/crypto/curve25519/const_amd64.h

@@ -0,0 +1,8 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This code was translated into a form compatible with 6a from the public
+// domain sources in SUPERCOP: https://bench.cr.yp.to/supercop.html
+
+#define REDMASK51     0x0007FFFFFFFFFFFF