add empty header for backwards compatibility.

BUG=none
TESTED=lint passes

Review URL: https://webrtc-codereview.appspot.com/46919004

git-svn-id: http://libyuv.googlecode.com/svn/trunk@1371 16f28f9a-4ce2-e073-06de-1de4eb20be90

include/libyuv/format_conversion.h

+/*
+ *  Copyright 2015 The LibYuv Project Authors. All rights reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS. All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+/* format_conversion.h has been deprecated. */
+/* Functionality for bayer formats (BGGR) removed. */
+
+#ifndef INCLUDE_LIBYUV_FORMAT_CONVERSION_H_  // NOLINT
+#define INCLUDE_LIBYUV_FORMAT_CONVERSION_H_
+#endif  // INCLUDE_LIBYUV_FORMAT_CONVERSION_H_
+