selftests: pid_namespace: Fix missing mount headers in pid_max.c

Fix compilation errors in pid_max.c by including <sys/mount.h>, which provides definitions for mount(), umount2(), MS_PRIVATE, MS_REC, and MNT_DETACH.

Without this header, the build fails with implicit declarations and undefined constants during selftest compilation.

Changes since v1:
- Included example build errors below for clarity

Compile error:
pid_max.c: In function ‘pid_max_cb’:
pid_max.c:42:15: warning: implicit declaration of function ‘mount’ [-Wimplicit-function-declaration]

   42 |         ret = mount("", "/", NULL, MS_PRIVATE | MS_REC, 0);
      |               ^~~~~

pid_max.c:42:36: error: ‘MS_PRIVATE’ undeclared; did you mean ‘MAP_PRIVATE’? pid_max.c:42:49: error: ‘MS_REC’ undeclared
pid_max.c:48:9: warning: implicit declaration of function ‘umount2’ pid_max.c:48:26: error: ‘MNT_DETACH’ undeclared

[...output trimmed for brevity...]

make: *** [../lib.mk:222: /linux_mainline/tools/testing/selftests/pid_namespace/pid_max] Error 1

---
 tools/testing/selftests/pid_namespace/pid_max.c | 1 +
 1 file changed, 1 insertion(+)

index 51c414faabb0..972bedc475f1 100644
@@ -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