hpke-rs-rust-crypto: make deterministic-prng enable the std feature

[japaric]

I noticed that running cargo build --all-features from the rust_crypto_provider directory does not work unless the std feature of its hpke-rs-crypto dependency is enabled so this PR fixes that

this is not a problem when running cargo test from the root of the repo because hpke-rs enables both hpke-rs-rust-crypto/deterministic-prng and hpke-rs-crypto. would it be worthwhile to run cargo b --all-features from the rust_crypto_provider directory in CI?

[franziskuskiefer]

would it be worthwhile to run cargo b --all-features from the rust_crypto_provider directory in CI?

Good point. Yes I think it would be worth doing that.