It is currently a `ipn.PrefsView` which means when we do a JSON roundtrip,
we go from an invalid Prefs to a valid one.
This makes it a pointer, which fixes the JSON roundtrip.
This was introduced in 0957bc5af2a544f7d197ea1e743696e8ba103b9f.
Signed-off-by: Maisem Ali <maisem@tailscale.com>