`update_self` fails to compile for more unusual targets

I happened to be working on my (android) phone using Termux, and ezno was throwing up an error because the `#[cfg(target_os = ...)]`s in `update_self` in `src/utilities.rs` don't cover every possible case. I fixed compilation by adding an additional cfg statement for `"android"` which allowed it to compile but probably doesn't work. I'd suggest that perhaps for the case where target_os is not windows, mac or linux that `update_self` is stubbed out for a no-op function so that things compile on slightly more exotoc platforms, although other, nicer solutions may exist.