btrfs: add Kconfig dependency for BLAKE2B

Because the BLAKE2B code went through a different tree, it was not
available at the time the btrfs part was merged. Now that the Kconfig
symbol exists, add it to the list.
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/Kconfig

@@ -7,6 +7,7 @@ config BTRFS_FS
 	select LIBCRC32C
 	select CRYPTO_XXHASH
 	select CRYPTO_SHA256
+	select CRYPTO_BLAKE2B
 	select ZLIB_INFLATE
 	select ZLIB_DEFLATE
 	select LZO_COMPRESS