Squashfs: fix function prototype

The fourth argument should be unsigned.  Also add missing include
so that the function prototype is defined in xattr_id.c

This fixes a couple of sparse warnings.

Signed-off-by: Phillip Lougher <phillip@lougher.demon.co.uk>
diff --git a/fs/squashfs/xattr_id.c b/fs/squashfs/xattr_id.c
index cfb4110..d33be5d 100644
--- a/fs/squashfs/xattr_id.c
+++ b/fs/squashfs/xattr_id.c
@@ -34,6 +34,7 @@
 #include "squashfs_fs_sb.h"
 #include "squashfs_fs_i.h"
 #include "squashfs.h"
+#include "xattr.h"
 
 /*
  * Map xattr id using the xattr id look up table