Move copyright to the top of files.

src/ftxui/dom/box_helper.cpp

@@ -1,3 +1,6 @@
+// Copyright 2021 Arthur Sonzogni. All rights reserved.
+// Use of this source code is governed by the MIT license that can be found in
+// the LICENSE file.
 #include "ftxui/dom/box_helper.hpp"
 
 #include <algorithm>  // for max
@@ -86,7 +89,3 @@ void Compute(std::vector<Element>* elements, int target_size) {
 }
 
 }  // namespace ftxui::box_helper
-
-// Copyright 2021 Arthur Sonzogni. All rights reserved.
-// Use of this source code is governed by the MIT license that can be found in
-// the LICENSE file.