Unsupported characters break XML files

I used addlicense to add license + copyright header to FLARE-VM in https://github.com/mandiant/flare-vm/pull/638. The `config.xml file` had a `` (Byte Order Mark (BOM)) that caused that addlicense added the header in the wrong way, breaking the file (and FLARE-VM). Support this type of characters to avoid issues in the future. There is a PR already with a proposed fixed in https://github.com/google/addlicense/pull/166.