download_sysext: fix clippy warning about needless_timeout

Fix warning when running cargo-clippy:

```
warning: the following explicit lifetimes could be elided: 'a
  --> src/bin/download_sysext.rs:47:6
   |
47 | impl<'a> Package<'a> {
   |      ^^          ^^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_lifetimes
   = note: `#[warn(clippy::needless_lifetimes)]` on by default
help: elide the lifetimes
   |
47 - impl<'a> Package<'a> {
47 + impl Package<'_> {
   |
```

src/bin/download_sysext.rs

@@ -44,7 +44,7 @@ struct Package<'a> {
     status: PackageStatus,
 }
 
-impl<'a> Package<'a> {
+impl Package<'_> {
     #[rustfmt::skip]
     // Return Sha256 hash of data in the given path.
     // If maxlen is None, a simple read to the end of the file.