vk_validation_stats: Update Copyright year

The header file was updated, but this script was not.
diff --git a/scripts/vk_validation_stats.py b/scripts/vk_validation_stats.py
index 3656731..7fbf0e4 100755
--- a/scripts/vk_validation_stats.py
+++ b/scripts/vk_validation_stats.py
@@ -479,8 +479,8 @@
 /*
  * Vulkan
  *
- * Copyright (c) 2016-2018 Google Inc.
- * Copyright (c) 2016-2018 LunarG, Inc.
+ * Copyright (c) 2016-2019 Google Inc.
+ * Copyright (c) 2016-2019 LunarG, Inc.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.