HookExecutor: Add missing licence header

Signed-off-by: Edwin Kempin <ekempin@google.com>
Change-Id: I036a6b1f246efbf42d0cd0a8555ef582e8a78293
diff --git a/src/main/java/com/googlesource/gerrit/plugins/hooks/HookExecutor.java b/src/main/java/com/googlesource/gerrit/plugins/hooks/HookExecutor.java
index 0232f9b..6e5ada5 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/hooks/HookExecutor.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/hooks/HookExecutor.java
@@ -1,3 +1,17 @@
+// Copyright (C) 2016 The Android Open Source Project
+// Licensed 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.
 package com.googlesource.gerrit.plugins.hooks;
 import com.google.common.flogger.FluentLogger;