Add a source line to the header, because type must always be fallback
The dir list spec says that fallback files start with:
/* type=fallback */
https://gitweb.torproject.org/torspec.git/tree/dir-list-spec.txt#n140
But in #24953 (moved), we change type to whitelist by default. This might cause some parsers to break.
Instead, we should add a new /* source=whitelist|fallback*/
line.