Add license header to mark-subsource-and-subflow-final excludes (#2542)

Added license information to the excludes file.

Author: pjfanning

stream/src/main/mima-filters/2.0.x.backwards.excludes/mark-subsource-and-subflow-final.backwards.excludes

@@ -1,3 +1,20 @@
+# 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.
+
 # Mark SubSource and SubFlow final class
 ProblemFilters.exclude[FinalClassProblem]("org.apache.pekko.stream.javadsl.SubSource")
 ProblemFilters.exclude[FinalClassProblem]("org.apache.pekko.stream.javadsl.SubFlow")