MinGW: js_of_ocaml complains missing implementations: `win_cleanup`, `win_handle_fd` and `win_startup`

[camlspotter]

Use of Unix.gettimeofday creates dependencies over C functions win_startup, win_cleanup and win_handle_fd.

Possible fix is to add them to unix.js:

diff --git a/runtime/unix.js b/runtime/unix.js
index a424c0d..49f2993 100644
--- a/runtime/unix.js
+++ b/runtime/unix.js
@@ -42,3 +42,19 @@ function unix_mktime(tm){
     var tm2 = unix_localtime(t);
     return [0,t,tm2];
 }
+
+//Provides: win_startup
+function win_startup()
+{
+}
+
+//Provides: win_cleanup
+function win_cleanup()
+{
+}
+
+//Provides: win_handle_fd
+function win_handle_fd(x)
+{
+  return x;
+}

I do not test win_handle_fd(). Please review it.