Make iana-time-zone a target specific dependency

Currently, iana-tiime-zone is only used on cfg(unix). This crate,
and its windows code in particular, contains a lot of unsafe, so it
seems prudent to limit its scope to where it is actually needed.

Cargo.toml

@@ -36,7 +36,6 @@ serde = { version = "1.0.99", default-features = false, optional = true }
 pure-rust-locales = { version = "0.5.2", optional = true }
 criterion = { version = "0.4.0", optional = true }
 rkyv = {version = "0.7", optional = true}
-iana-time-zone = { version = "0.1.45", optional = true, features = ["fallback"] }
 arbitrary = { version = "1.0.0", features = ["derive"], optional = true }
 
 [target.'cfg(all(target_arch = "wasm32", not(any(target_os = "emscripten", target_os = "wasi"))))'.dependencies]
@@ -47,6 +46,9 @@ js-sys = { version = "0.3", optional = true } # contains FFI bindings for the JS
 [target.'cfg(windows)'.dependencies]
 winapi = { version = "0.3.0", features = ["std", "minwinbase", "minwindef", "timezoneapi"], optional = true }
 
+[target.'cfg(unix)'.dependencies]
+iana-time-zone = { version = "0.1.45", optional = true, features = ["fallback"] }
+
 [dev-dependencies]
 serde_json = { version = "1" }
 serde_derive = { version = "1", default-features = false }