Squashfs: add XZ compression support

Add support for reading file systems compressed with the
XZ compression algorithm.

This patch adds the XZ decompressor wrapper code.

Signed-off-by: Phillip Lougher <phillip@lougher.demon.co.uk>
diff --git a/fs/squashfs/squashfs_fs.h b/fs/squashfs/squashfs_fs.h
index c5137fc..39533fe 100644
--- a/fs/squashfs/squashfs_fs.h
+++ b/fs/squashfs/squashfs_fs.h
@@ -238,6 +238,7 @@
 #define ZLIB_COMPRESSION	1
 #define LZMA_COMPRESSION	2
 #define LZO_COMPRESSION		3
+#define XZ_COMPRESSION		4
 
 struct squashfs_super_block {
 	__le32			s_magic;