journald: disable default features of tracing-subscriber (#1476)

## Motivation

Closes #1465.

## Solution

I'm just disabling the default features of `tracing-journald`'s
dependency on `tracing-subscriber`. The original issue talked about the
crate's dependencies more broadly but considering that the standard
library is already depended upon I didn't think it made sense to change
the `tracing-core` dependency's features (which are just `no_std`
support).

tracing-journald/Cargo.toml

@@ -18,9 +18,9 @@ rust-version = "1.49.0"
 [dependencies]
 libc = "0.2.107"
 tracing-core = { path = "../tracing-core", version = "0.1.10" }
-tracing-subscriber = { path = "../tracing-subscriber", version = "0.3" }
+tracing-subscriber = { path = "../tracing-subscriber", version = "0.3", default-features = false, features = ["registry"] }
 
 [dev-dependencies]
 serde_json = "1.0.68"
 serde = { version = "1.0.130", features = ["derive"] }
-tracing = { path = "../tracing", version = "0.1" }
+tracing = { path = "../tracing", version = "0.1" }