Unbreak the Windows build

[sylvestre]

fails with:

error[E0432]: unresolved import `libc::PRIO_PROCESS`
 --> src\uu\renice\src/renice.rs:6:5
  |
6 | use libc::PRIO_PROCESS;
  |     ^^^^^^^^^^^^^^^^^^ no `PRIO_PROCESS` in the root

error[E0425]: cannot find function `setpriority` in crate `libc`
  --> src\uu\renice\src/renice.rs:31:23
   |
31 |     if unsafe { libc::setpriority(PRIO_PROCESS, pid.try_into().unwrap(), nice_value) } == -1 {
   |                       ^^^^^^^^^^^ not found in `libc`

[cakebaker]

"Fixed" in #27