selftests/landlock: Fix build of audit_test

We are hitting build error on CentOS 9:

audit_test.c:232:40: error: ‘O_CLOEXEC’ undeclared (...)

Fix this by including fcntl.h.

Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20250605214416.1885878-1-song@kernel.org
Fixes: 6b45664 ("selftests/landlock: Add PID tests for audit records")
Signed-off-by: Mickaël Salaün <mic@digikod.net>

Author: liu-song-6

tools/testing/selftests/landlock/audit_test.c

@@ -7,6 +7,7 @@
 
 #define _GNU_SOURCE
 #include <errno.h>
+#include <fcntl.h>
 #include <limits.h>
 #include <linux/landlock.h>
 #include <pthread.h>