Build failure when specifying elf32 feature without elf64

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Disable default-features and specify only `elf32`:

```toml
[dependencies]
goblin = { version = "0.2", default-features = false, features = ["std", "elf32"] }
```

Build fails:

```
Compiling goblin v0.2.0
error[E0432]: unresolved import `crate::elf64`
--> /Users/parasyte/.cargo/registry/src/github.com-1ecc6299db9ec823/goblin-0.2.0/src/elf/header.rs:207:17
|
207 | use crate::elf64;
| ^^^^^^^^^^^^ no `elf64` in the root

error[E0433]: failed to resolve: maybe a missing crate `elf64`?
--> /Users/parasyte/.cargo/registry/src/github.com-1ecc6299db9ec823/goblin-0.2.0/src/elf/dynamic.rs:704:35
|
704 | elf_dyn_std_impl!(u64, crate::elf64::program_header::ProgramHeader);
| ^^^^^ maybe a missing crate `elf64`?

error: aborting due to 2 previous errors

Some errors have detailed explanations: E0432, E0433.
For more information about an error, try `rustc --explain E0432`.
error: could not compile `goblin`.
```

Looking at the code, specifying the opposite (`elf64` without `elf32`) will also fail for similar reasons.

Reproduce the failure with the shown Cargo.toml feature configuration, then inspect src/elf/header.rs and src/elf/dynamic.rs, the files named in the compiler errors. Verify the fix by building with only elf32 and only elf64 enabled; done means both configurations compile without unresolved elf module references.

Written by the indexing model from the issue text.