missing include in hppfs

Signed-off-by: Al Viro <[e-mail]>

fs/hppfs/hppfs.c

···
 #include <linux/slab.h>
 #include <linux/statfs.h>
 #include <linux/types.h>
+#include <linux/pid_namespace.h>
 #include <asm/uaccess.h>
 #include "os.h"