cksum: enable encoding feature

[cakebaker]

Running cargo test --features "cksum" --no-default-features currently leads to the following error:

error[E0432]: unresolved import `uucore::encoding`
  --> src/uu/cksum/src/cksum.rs:18:5
   |
18 |     encoding,
   |     ^^^^^^^^ no `encoding` in the root
   |
note: found an item that was configured out
  --> /home/dho/projects/coreutils/src/uucore/src/lib/lib.rs:43:26
   |
43 | pub use crate::features::encoding;
   |                          ^^^^^^^^
   = note: the item is gated behind the `encoding` feature

This PR fixes the issue by enabling the encoding feature.

[sylvestre]

we really should add CI for this :/