[gjs] arg: Update copyright statement

[Philip Chimento <pchimento src gnome org>]

commit 841978865c7fd9a8ff3564ac8cfa31faae8ba0d1
Author: Marco Trevisan (Treviño) <mail 3v1n0 net>
Date:   Thu May 14 20:45:53 2020 +0200

    arg: Update copyright statement
    
    (Philip: For context, I am committing this in advance of GNOME/gjs!432
    as I go through the commits there one by one. A substantial part of this
    file is being rewritten by Marco)

 gi/arg.cpp | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/gi/arg.cpp b/gi/arg.cpp
index b53ff7f4..94fb7fff 100644
--- a/gi/arg.cpp
+++ b/gi/arg.cpp
@@ -1,6 +1,7 @@
 /* -*- mode: C++; c-basic-offset: 4; indent-tabs-mode: nil; -*- */
 /*
  * Copyright (c) 2008  litl, LLC
+ * Copyright (c) 2020  Canonical, Ltd.
  *
  * Permission is hereby granted, free of charge, to any person obtaining a copy
  * of this software and associated documentation files (the "Software"), to