Fix building issue

[BaoshanPang]

I saw this error when building rust for vxWorks targets:
/folk/prj-rust-dev/bpang/ala-diab-pb19l/latest/latest/.cargo/registry/src/git.luolix.top-1ecc6299db9ec823/rand_core-0.5.1/src/err
or.rs:150:28
|
150 | Error { inner: Box::new(error) }
| ^^^^^^^^^^^^^^^ the trait std::error::Error is not implemented for getrandom::error::Error
|
= note: required for the cast to the object type dyn std::error::Error + std::marker::Send + std::marker::Sync

And after adding "mod error_impls;" the issue disappears.

r? @newpavlov
cc @n-salim

[josephlr]

@BaoshanPang thanks for opening this. This is an error caused by my change. Essentially when feature = "std" and target_os = "vxworks", we don't add error_impls when we should.

Ideally we would fix the cfg_if declaration so that mod error_impls is included when feature = "std" is set, instead of this change, but that's going to make things messier. So I guess this change is OK.

Note that we're going to remove the automatic error_impls inclusion in 0.2. However, this shouldn't affect you as you're only using getrandom though rand/rand_core which sets the features correct regardless.

Can you rebase this change onto the current master, then I can approve and merge it.

[josephlr]

This should just be part of #86, as that PR hasn't merged yet. However, this is an important point, and should be accounted for in review.