Update src/Data/List/Fresh.agda

Author: jamesmckinna

src/Data/List/Fresh.agda

@@ -23,7 +23,7 @@ open import Data.Maybe.Base as Maybe using (Maybe; just; nothing)
 open import Data.Nat.Base using (ℕ; zero; suc)
 open import Function.Base using (_∘′_; flip; id; _on_)
 open import Relation.Nullary using (does)
-open import Relation.Unary as U using (Pred)
+open import Relation.Unary as Unary using (Pred; Decidable)
 open import Relation.Binary.Core using (Rel; REL)
 import Relation.Binary.Definitions as B using (Reflexive)
 open import Relation.Nary using (_⇒_; ∀[_])