wycheproof2blb: fix blobby import

[kpp]

Fix error:

   Compiling wycheproof2blb v0.2.0-pre (/home/kpp/rustcrypto_utils/wycheproof2blb)
error[E0425]: cannot find function `encode_blobs` in crate `blobby`
   --> wycheproof2blb/src/main.rs:155:33
    |
155 |     let (blb_data, _) = blobby::encode_blobs(&blobs);
    |                                 ^^^^^^^^^^^^ not found in `blobby`
    |
note: found an item that was configured out
   --> /home/kpp/utils/blobby/src/lib.rs:20:7
    |
    = note: the item is gated behind the `alloc` feature

I don't know why your CI doesn't catch it 🤷

[newpavlov]

I don't know why your CI doesn't catch it

Hm, it's probably because the feature gets enabled by another crate in the workspace.