Btrfs: select XOR_BLOCKS in Kconfig

The Btrfs raid56 uses the generic xor helpers.
Signed-off-by: NChris Mason <chris.mason@fusionio.com>

fs/btrfs/Kconfig

@@ -7,6 +7,7 @@ config BTRFS_FS
 	select LZO_COMPRESS
 	select LZO_DECOMPRESS
 	select RAID6_PQ
+	select XOR_BLOCKS
 
 	help
 	  Btrfs is a new filesystem with extents, writable snapshotting,