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

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/20250115105211.390370-3-ps.report@gmx.net
Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Reviewed-by: T.J. Mercier <tjmercier@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>