Building with clang results in the following warning:

  posix_timers.c:69:6: warning: absolute value function 'abs' given an
      argument of type 'long long' but has parameter of type 'int' which may
      cause truncation of value [-Wabsolute-value]
        if (abs(diff - DELAY * USECS_PER_SEC) > USECS_PER_SEC / 2) {
            ^
So switch to using llabs() instead.

Fixes: 0bc4b0cf ("selftests: add basic posix timers selftests")
Signed-off-by: John Stultz <jstultz@google.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20240410232637.4135564-3-jstultz@google.com