[PATCH RESEND] selftests: add missing include

[Chen Linxuan]

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@xxxxxxxxxxxxx>
---
 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 96f274f0582b6..c3ee8a2eff75b 100644
--- a/tools/testing/selftests/pid_namespace/pid_max.c
+++ b/tools/testing/selftests/pid_namespace/pid_max.c
@@ -12,6 +12,7 @@
 #include <syscall.h>
 #include <sys/mount.h>
 #include <sys/wait.h>
+#include <sys/mount.h>
 
 #include "../kselftest_harness.h"
 #include "../pidfd/pidfd.h"
-- 
2.43.0