set default file encoding to utf-8

There's no reason to support any other encoding in these files.
This only affects the files themselves and not streams they open.

Bug: https://crbug.com/gerrit/10418
Change-Id: I053cb40cd3666ce5c8a0689b9dd938f24ca765bf
diff --git a/platform_utils.py b/platform_utils.py
index 8af25d2..77e8fdf 100644
--- a/platform_utils.py
+++ b/platform_utils.py
@@ -1,3 +1,4 @@
+# -*- coding:utf-8 -*-
 #
 # Copyright (C) 2016 The Android Open Source Project
 #