Fix: multiple definition of 'polyHashADInternal'

- unlike https://github.com/Psiphon-Labs/psiphon-tunnel-core/commit/c36ea33b5439856f34a38ba27937b466be00f7ac,
  disable for just mingw

psiphon/common/crypto/chacha20poly1305/chacha20poly1305_amd64.s

@@ -4,7 +4,10 @@
 
 // This file was originally from https://golang.org/cl/24717 by Vlad Krasnov of CloudFlare.
 
-// +build go1.7,amd64,!gccgo,!appengine
+// [Psiphon]
+// disable for windows builds due to multiple definition error in mingw
+
+// +build go1.7,amd64,!gccgo,!appengine,!windows
 
 #include "textflag.h"
 // General register allocation