crypto: xts - Add softdep on ecb

The xts module needs ecb to be present as it's meant to work
on top of ecb.  This patch adds a softdep so ecb can be included
automatically into the initramfs.

Reported-by: rftc <rftc@gmx.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>