Btrfs: select XOR_BLOCKS in Kconfig

The Btrfs raid56 uses the generic xor helpers.

Signed-off-by: Chris Mason <chris.mason@fusionio.com>
diff --git a/fs/btrfs/Kconfig b/fs/btrfs/Kconfig
index 4f5dc93..5f583c8 100644
--- a/fs/btrfs/Kconfig
+++ b/fs/btrfs/Kconfig
@@ -7,6 +7,7 @@
 	select LZO_COMPRESS
 	select LZO_DECOMPRESS
 	select RAID6_PQ
+	select XOR_BLOCKS
 
 	help
 	  Btrfs is a new filesystem with extents, writable snapshotting,