[NaCl SDK] Fix SDK waterfall: unistd.h does not exist on windows.

native_client_sdk/src/libraries/nacl_io/osunistd.h

Index: native_client_sdk/src/libraries/nacl_io/osunistd.h
diff --git a/native_client_sdk/src/libraries/nacl_io/osunistd.h b/native_client_sdk/src/libraries/nacl_io/osunistd.h
new file mode 100644
index 0000000000000000000000000000000000000000..c5da0efe0a809f1a11bafdbfd0142f847f711e66
--- /dev/null
+++ b/native_client_sdk/src/libraries/nacl_io/osunistd.h
@@ -0,0 +1,22 @@
+/* Copyright (c) 2013 The Chromium Authors. All rights reserved.
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+#ifndef LIBRARIES_NACL_IO_OSUNISTD_H_
+#define LIBRARIES_NACL_IO_OSUNISTD_H_
+
+#if defined(WIN32)
+
+#define R_OK 4
+#define W_OK 2
+#define X_OK 1
+#define F_OK 0
+
+#else
+
+#include <unistd.h>
+
+#endif
+
+#endif  // LIBRARIES_NACL_IO_OSUNISTD_H_
+