selftests: pid_namespace: add missing sys/mount.h include in pid_max.c

Fix compile on openSUSE Tumbleweed (gcc-14.2.1, glibc-2.40):
- add missing sys/mount.h include

Fixes:

pid_max.c: In function ‘pid_max_cb’:
pid_max.c:42:15: error: implicit declaration of function ‘mount’ [-Wimplicit-function-declaration]
42 | ret = mount("", "/", NULL, MS_PRIVATE | MS_REC, 0);
| ^~~~~

Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Peter Seiderer <[e-mail]>
Reviewed-by: T.J. Mercier <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/pid_namespace/pid_max.c

···
 #include <stdlib.h>
 #include <string.h>
 #include <syscall.h>
+#include <sys/mount.h>
 #include <sys/wait.h>
 
 #include "../kselftest_harness.h"