Squashfs: Add LZ4 compression configuration option

Add the glue code, and also update the documentation.

Signed-off-by: Phillip Lougher <phillip@squashfs.org.uk>
diff --git a/fs/squashfs/decompressor.h b/fs/squashfs/decompressor.h
index af09853..a25713c 100644
--- a/fs/squashfs/decompressor.h
+++ b/fs/squashfs/decompressor.h
@@ -46,6 +46,10 @@
 extern const struct squashfs_decompressor squashfs_xz_comp_ops;
 #endif
 
+#ifdef CONFIG_SQUASHFS_LZ4
+extern const struct squashfs_decompressor squashfs_lz4_comp_ops;
+#endif
+
 #ifdef CONFIG_SQUASHFS_LZO
 extern const struct squashfs_decompressor squashfs_lzo_comp_ops;
 #endif