Address compilation warnings

[ Merge of http://go/wvgerrit/119523 ]

Bug: 182299187
Test: compile vendor/widevine/libwvdrmengine
Change-Id: I39d1b62a3237bfb9163a7e157b871f22e3fec394

libwvdrmengine/cdm/util/include/wv_attributes.h

@@ -0,0 +1,16 @@
+// Copyright 2021 Google LLC. All rights reserved. This file and proprietary
+// source code may only be used and distributed under the Widevine
+// License Agreement.
+
+#ifndef WVCDM_UTIL_WV_ATTRIBUTES_H_
+#define WVCDM_UTIL_WV_ATTRIBUTES_H_
+
+#ifndef UNUSED
+#  if defined(__GNUC__) || defined(__clang__)
+#    define UNUSED __attribute__((__unused__))
+#  else
+#    define UNUSED
+#  endif
+#endif
+
+#endif  // WVCDM_UTIL_WV_ATTRIBUTES_H_