I failed to build this test on Ubuntu 24.04.
Compiler complained about undefined functions mount, umount and some
mount related flags.

Signed-off-by: Chen Linxuan <chenlinxuan@uniontech.com>
---
 tools/testing/selftests/pid_namespace/pid_max.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/tools/testing/selftests/pid_namespace/pid_max.c b/tools/testing/selftests/pid_namespace/pid_max.c
index 51c414faabb0f..972bedc475f12 100644
--- a/tools/testing/selftests/pid_namespace/pid_max.c
+++ b/tools/testing/selftests/pid_namespace/pid_max.c
@@ -11,6 +11,7 @@
 #include <string.h>
 #include <syscall.h>
 #include <sys/wait.h>
+#include <sys/mount.h>
 
 #include "../kselftest_harness.h"
 #include "../pidfd/pidfd.h"
-- 
2.43.0