include/posix/io_posix.h should have a once declartion

Summary: include/posix/io_posix.h doesn't not prevent multiple includes. Need to fix it. It is also breaking unity build.

Test Plan: Run unity build and see error go away.

Reviewers: rven, igor, IslamAbdelRahman, kradhakrishnan, anthony

Subscribers: leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D49281

include/posix/io_posix.h

@@ -6,6 +6,7 @@
 // Copyright (c) 2011 The LevelDB Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file. See the AUTHORS file for names of contributors.
+#pragma once
 #include <deque>
 #include <set>
 #include <dirent.h>