Always import cc, even if it's useless

This is required for `build.rs`, even if it returns right away
because the pure rust versions will be compiled.

Ideally, we should not import cc at least if `pure-rust` is defined,
but `cargo` doesn't seem to have any ways to do that, and `#[cfg]`
macros wouldn't work in `build.rs` anyway.

Cargo.toml

@@ -14,7 +14,7 @@ readme = "README.md"
 [target."cfg(not(any(all(target_arch = \"aarch64\", target_feature = \"aes\"),all(target_arch = \"x86_64\", target_feature = \"aes\"),all(target_arch = \"x86\", target_feature = \"aes\"))))".dependencies.softaes]
 version = "0.1"
 
-[target.'cfg(not(pure_rust))'.build-dependencies]
+[build-dependencies]
 cc = "1.0.83"
 
 [dev-dependencies]