Adde LICENSE Header to build/mvn, build/sbt and sbt/sbt

Recently, build/mvn and build/sbt are added, and sbt/sbt is changed but there are no license headers. Should we add license headers to the scripts right?
If it's not right, please let me correct.

This PR doesn't affect behavior of Spark, I don't file in JIRA.

Author: Kousuke Saruta <sarutak@oss.nttdata.co.jp>

Closes #3817 from sarutak/add-license-header and squashes the following commits:

1abc972 [Kousuke Saruta] Added LICENSE Header

1 files changed, 17 insertions, 0 deletions

diff --git a/build/sbt b/build/sbt
index 0a251d97db..28ebb64f71 100755
--- a/build/sbt
+++ b/build/sbt
@@ -1,5 +1,22 @@
 #!/usr/bin/env bash
 
+#
+# 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.
+#
+
 # When creating new tests for Spark SQL Hive, the HADOOP_CLASSPATH must contain the hive jars so
 # that we can run Hive to generate the golden answer.  This is not required for normal development
 # or testing.