selftests: kselftest: Fix build failure with NOLIBC

As Mark explains ksft_min_kernel_version() can't be compiled with nolibc,
it doesn't implement uname().

Fixes: 6d029c25

 ("selftests/timers/posix_timers: Reimplement check_timer_distribution()")
Reported-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240412123536.GA32444@redhat.com
Closes: https://lore.kernel.org/all/f0523b3a-ea08-4615-b0fb-5b504a2d39df@sirena.org.uk/