pidfs/selftests: ensure correct headers for ioctl handling

Ensure that necessary ioctl infrastructure is available.

Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Christian Brauner <[e-mail]>

tools/testing/selftests/pidfd/pidfd_setns_test.c

···
 #include <unistd.h>
 #include <sys/socket.h>
 #include <sys/stat.h>
-#include <linux/ioctl.h>
+#include <sys/ioctl.h>
 
 #include "pidfd.h"
 #include "../kselftest_harness.h"