lints: Add a missing copyright header

Co-authored-by: Daira Hopwood <daira@jacaranda.org>

test/lint/lint-shebang.sh

@@ -1,4 +1,9 @@
 #!/usr/bin/env bash
+#
+# Copyright (c) 2018 The Bitcoin Core developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or https://www.opensource.org/licenses/mit-license.php .
+#
 # Assert expected shebang lines
 
 export LC_ALL=C