missing include in hppfs

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/hppfs/hppfs.c

@@ -15,6 +15,7 @@
 #include <linux/slab.h>
 #include <linux/statfs.h>
 #include <linux/types.h>
+#include <linux/pid_namespace.h>
 #include <asm/uaccess.h>
 #include "os.h"