Adding license header to check_license.sh script.

Signed-off-by: Tim 'mithro' Ansell <tansell@google.com>

.github/check_license.sh

@@ -1,4 +1,11 @@
-#!/bin/bash
+#!/usr/bin/env bash
+# Copyright (C) 2017-2020  The Project X-Ray Authors.
+#
+# Use of this source code is governed by a ISC-style
+# license that can be found in the LICENSE file or at
+# https://opensource.org/licenses/ISC
+#
+# SPDX-License-Identifier: ISC
 
 echo
 echo "==========================="