Make gw_ds_complete less public

[mkeeter]

I'm a little confused by this – crucible compiles just fine, but using it in propolis-standalone gives a compile-time error message:

error[E0446]: crate-private type `GuestWorkId` in public interface
   --> /home/matt/crucible/upstairs/src/guest.rs:246:5
    |
150 | pub(crate) struct GuestWorkId(pub u64);
    | ----------------------------- `GuestWorkId` declared as crate-private
...
246 |     #[instrument]
    |     ^^^^^^^^^^^^^ can't leak crate-private type
    |
    = note: this error originates in the attribute macro `instrument` (in Nightly builds, run with -Z macro-backtrace for more info)

For more information about this error, try `rustc --explain E0446`.

[mkeeter]

Update: it turns out that changing gw_ds_complete from pub to pub(crate) also resolves the issue, so let's do that instead!