lock_file_exemptions: add ASF license header

RAT (Release Audit Tool) check on apache/infrastructure-actions requires
all source files to carry the Apache License v2 header. The new
lock_file_exemptions.yml was missing it, blocking the rat check on #770.
Spotted by @dave2wave.

Author: potiuk

lock_file_exemptions.yml

@@ -1,3 +1,21 @@
+#
+# Licensed to the Apache Software Foundation (ASF) under one
+# or more contributor license agreements.  See the NOTICE file
+# distributed with this work for additional information
+# regarding copyright ownership.  The ASF licenses this file
+# to you under the Apache License, Version 2.0 (the
+# "License"); you may not use this file except in compliance
+# with the License.  You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing,
+# software distributed under the License is distributed on an
+# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+# KIND, either express or implied.  See the License for the
+# specific language governing permissions and limitations
+# under the License.
+#
 # lock_file_exemptions.yml
 #
 # Per-(org/repo) per-ecosystem exemptions from the "every dependency manifest