Fix missing import

Author: MatthewDaggitt

src/Data/Vec/Base.agda

@@ -8,7 +8,7 @@
 
 module Data.Vec.Base where
 
-open import Data.Bool.Base using (Bool; true; false)
+open import Data.Bool.Base using (Bool; true; false; if_then_else_)
 open import Data.Nat.Base
 open import Data.Fin.Base using (Fin; zero; suc)
 open import Data.List.Base as List using (List)