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: 6d029c25b71f ("selftests/timers/posix_timers: Reimplement check_timer_distribution()")
Reported-by: Mark Brown <[e-mail]>
Signed-off-by: Oleg Nesterov <[e-mail]>
Signed-off-by: Thomas Gleixner <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Closes: https://lore.kernel.org/all/[e-mail]/

tools/testing/selftests/kselftest.h

···
 static inline int ksft_min_kernel_version(unsigned int min_major,
 					  unsigned int min_minor)
 {
+#ifdef NOLIBC
+	ksft_print_msg("NOLIBC: Can't check kernel version: Function not implemented\n");
+	return 0;
+#else
 	unsigned int major, minor;
 	struct utsname info;
 
···
 		ksft_exit_fail_msg("Can't parse kernel version\n");
 
 	return major > min_major || (major == min_major && minor >= min_minor);
+#endif
 }
 
 #endif /* __KSELFTEST_H */