Fix std-ness

Author: pnehrer

Cargo.toml

@@ -19,14 +19,14 @@ rust-version = "1.65"
 
 [features]
 default = ["std"]
-std = ["slog/std"]
+std = ["slog/std", "once_cell/std", "tracing-core/std"]
 # Enables serialization of slog key-value pairs to a constructed field, `slog.kv`
 kv = []
 
 [dependencies]
-once_cell = "1"
-slog = "2.8"
-tracing-core = "0.1"
+once_cell = { version = "1", default-features = false }
+slog = { version = "2.8", default-features = false }
+tracing-core = { version = "0.1", default-features = false }
 
 [dev-dependencies]
 log = "0.4"