Fix getopt signature in port_win.h

src/shared/platform/win/port_win.c

 /*
  * Copyright 2008 The Native Client Authors. All rights reserved.
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 /*
  * Misc functions missing from windows.
  */
 #include <stdio.h>
@@ skipping 51 matching lines @@
 
 
 /*
  * Based on code by Hans Dietrich
  * see http://www.codeproject.com/KB/cpp/xgetopt.aspx
  */
 
 char *optarg;  /* global argument pointer */
 int optind = 0;  /* global argv index */
 
-int getopt(int argc, char *argv[], char *optstring) {
+int getopt(int argc, char *argv[], const char *optstring) {
   char c, *cp;
   static char *next = NULL;
   if (optind == 0)
     next = NULL;
 
   optarg = NULL;
 
   if (next == NULL || *next == ('\0')) {
     if (optind == 0)
       optind++;
@@ skipping 34 matching lines @@
     } else if (optind < argc) {
       optarg = argv[optind];
       optind++;
     } else {
       return ('?');
     }
   }
 
   return c;
 }