Add license line

Signed-off-by: Andy Lok <andylokandy@hotmail.com>

Author: andylokandy

.gitignore

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 target
+.vscode

.vscode/settings.json

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+// Copyright 2021 TiKV Project Authors. Licensed under Apache-2.0.
+
 #include "tikv_client.h"
 #include <iostream>
 

example/main.cpp

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+// Copyright 2021 TiKV Project Authors. Licensed under Apache-2.0.
+
 #ifndef _TIKV_CLIENT_H_
 #define _TIKV_CLIENT_H_ 
 

include/tikv_client.h

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+// Copyright 2021 TiKV Project Authors. Licensed under Apache-2.0.
+
 use core::panic;
 use std::ops;
 

src/lib.rs

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+// Copyright 2021 TiKV Project Authors. Licensed under Apache-2.0.
+
 #include "tikv_client.h"
 
 using namespace std;