Added copyright.

src/gcc_atomic.cpp

+//
+// MessagePack for C++ atomic operations
+//
+// Copyright (C) 2008-2013 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.
+//
+
 #if defined(__GNUC__) && ((__GNUC__*10 + __GNUC_MINOR__) < 41)
 
 #include "gcc_atomic.h"