book: Include ashpd only on linux so that listings run on macos

[daxhuiberts]

This will make ashpd only a dependency on linux.

An alternative would be something like cfg(all(unix, not(target_os = "macos"))) to still include it for all other unix family.

[bilelmoussaoui]

The code using it should also be behind a similar guard

[daxhuiberts]

The code using it should also be behind a similar guard

I looked into disabling the main_event_loop_7 bin, but there currently is no simple way to achieve that (see rust-lang/cargo#9208)
Not sure what the appropriate approach would be in this case.

[daxhuiberts]

I made a change to main_event_loop_7 with a possible solution, but this approach is probably not desirable.

[bilelmoussaoui]

Also, please fix the formatting, otherwise the suggested solution is the best we can do so it is fine.

[daxhuiberts]

Thanks for the feedback.
Fixed the newline and formatting.

[bilelmoussaoui]

Thanks!