Fix some copyright lines in folly/experimental/symbolizer/

Summary:
[Folly] Fix some copyright lines in `folly/experimental/symbolizer/`.

(Note: this ignores all push blocking failures!)

Reviewed By: Orvid

Differential Revision: D6648183

fbshipit-source-id: 36f9df5af91400a37dfa5ee2b209ffd47d5069df

folly/experimental/symbolizer/SignalHandler.cpp

 /*
- * Copyright 2017 Facebook, Inc.
+ * Copyright 2013-present Facebook, Inc.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.

folly/experimental/symbolizer/SignalHandler.h

 /*
- * Copyright 2017 Facebook, Inc.
+ * Copyright 2013-present Facebook, Inc.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.

folly/experimental/symbolizer/Symbolizer.cpp

 /*
- * Copyright 2017 Facebook, Inc.
+ * Copyright 2012-present Facebook, Inc.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.

folly/experimental/symbolizer/Symbolizer.h

 /*
- * Copyright 2017 Facebook, Inc.
+ * Copyright 2012-present Facebook, Inc.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.