Fix explicit imports test (#513)

Add `@horner` to the list of functions which are imported though not public.

Author: nalimilan

test/qa.jl

@@ -26,6 +26,7 @@ end
             # Ref https://github.com/JuliaTesting/ExplicitImports.jl/issues/92
             :invπ, # SpecialFunctions
             :sqrtπ, # SpecialFunctions
+            Symbol("@horner"), # Base.Math
         ),
     ) === nothing