sync: move call to unsafe fn to unsafe block

[cakebaker]

I noticed the following warning in the Android jobs of the CI (see, for example, https://github.com/uutils/coreutils/actions/runs/15039807606/job/42268624578#step:16:836):

[2025-05-15 08:08:33] warning[E0133]: call to unsafe function `libc::syscall` is unsafe and requires unsafe block
[2025-05-15 08:08:33]   --> src/uu/sync/src/sync.rs:45:9
[2025-05-15 08:08:33]    |
[2025-05-15 08:08:33] 45 |         libc::syscall(libc::SYS_sync);
[2025-05-15 08:08:33]    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ call to unsafe function

This PR fixes this warning by moving the function call inside the already existing unsafe block used for the non-Android code.

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/timeout/timeout (passes in this run but fails in the 'main' branch)