uucore: add chrono to uptime feature

[cakebaker]

Running cargo test --features=uptime --no-default-features currently leads to an "unresolved import chrono" error:

error[E0432]: unresolved import `chrono`
  --> src/uucore/src/lib/features/uptime.rs:16:5
   |
16 | use chrono::Local;
   |     ^^^^^^ use of unresolved module or unlinked crate `chrono`

This PR fixes this issue by adding chrono to the uptime feature.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/misc/stdbuf (fails in this run but passes in the 'main' branch)