scripted-diff: Insert missed copyright header

-BEGIN VERIFY SCRIPT-
./contrib/devtools/copyright_header.py insert src/policy/fees_args.cpp
-END VERIFY SCRIPT-

Author: hebasto

src/policy/fees_args.cpp

@@ -1,3 +1,7 @@
+// Copyright (c) 2021 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
 #include <policy/fees_args.h>
 
 #include <util/system.h>