selftests/pidfd: fix header inclusion

Ensure that necessary defines are present.

Link: https://lore.kernel.org/r/20250305-work-pidfs-kill_on_last_close-v3-7-c8c3d8361705@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>

tools/testing/selftests/pidfd/pidfd_fdinfo_test.c

@@ -13,6 +13,7 @@
 #include <syscall.h>
 #include <sys/wait.h>
 #include <sys/mman.h>
+#include <sys/mount.h>
 
 #include "pidfd.h"
 #include "../kselftest.h"