sys: clock: re-apply change to Z_IS_TIMEOUT_RELATIVE

A mid-air collision between two different changes to sys_clock.h
resulted in a test failure.

```shell
tests/kernel/timer/timer_api/src/main.c:763:
  timer_api_test_timeout_abs: Z_IS_TIMEOUT_RELATIVE(t2) is false
```

Re-apply the change that was added in

9075d533

to keep tests running smoothly.

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>