How to handle cbits part of this package?

Hi, I can't seem to build cardano-crypto-class in plain nix (not haskell.nix), due to the cbits library.

I have a cabal2nix expression for cardano-crypto-class and am overriding it with addPkgconfigDepends *and* addExtraLibraries, and supplying the argument.

```
cardano-crypto-class = addPkgconfigDepends (addExtraLibraries (prev.callPackage ./pkgs/cardano-crypto-class.nix { inherit libsodium libblst secp256k1;}) [libblst]) [libblst];
```

It still results in

```
error: builder for '/nix/store/rm7kkf407l8k4bph7d217b1qjb6dlwkv-cardano-crypto-class-2.1.2.0.drv' failed with exit code 1;
last 10 log lines:
>
> cbits/blst_util.c:1:10: error:
> fatal error: blst.h: No such file or directory
> 1 | #include "blst.h"
> | ^~~~~~~~
> |
> 1 | #include "blst.h"
> | ^
> compilation terminated.
> `gcc' failed in phase `C Compiler'. (Exit code: 1)
```

I'm pulling libblst from iohk-nix/26f56e32169dcc9ef72ac754eccdb3c96d714751. How do I tell the package where the headers are?