Fixed -Wextra warnings on gcc.

include/msgpack/util.h

@@ -0,0 +1,23 @@
+/*
+ * MessagePack for C utilities
+ *
+ * Copyright (C) 2014 FURUHASHI Sadayuki
+ *
+ *    Licensed under the Apache License, Version 2.0 (the "License");
+ *    you may not use this file except in compliance with the License.
+ *    You may obtain a copy of the License at
+ *
+ *        http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *    Unless required by applicable law or agreed to in writing, software
+ *    distributed under the License is distributed on an "AS IS" BASIS,
+ *    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *    See the License for the specific language governing permissions and
+ *    limitations under the License.
+ */
+#ifndef MSGPACK_UTIL_H
+#define MSGPACK_UTIL_H
+
+#define MSGPACK_UNUSED(a) (void)(a)
+
+#endif /* MSGPACK_UTIL_H */