[variadic] only emit "untransformed" message for builtins that are variadic.

src/plugins/variadic/tests/erroneous/oracle/variadic-builtin.res.oracle

+[variadic] tests/erroneous/variadic-builtin.i:1: 
+  Declaration of variadic function Frama_C_show_each_warning.
 [variadic] tests/erroneous/variadic-builtin.i:1: 
   Variadic builtin Frama_C_show_each_warning left untransformed.
 [kernel] tests/erroneous/variadic-builtin.i:6: Plug-in variadic aborted: unimplemented feature.

src/plugins/variadic/tests/known/oracle/printf_garbled_mix.res.oracle

@@ -14,8 +14,6 @@
   Declaration of variadic function sscanf.
 [variadic] FRAMAC_SHARE/libc/stdio.h:541: 
   Declaration of variadic function dprintf.
-[variadic] tests/known/printf_garbled_mix.c:8: 
-  Variadic builtin Frama_C_show_each_nb_printed left untransformed.
 [variadic] tests/known/printf_garbled_mix.c:7: 
   Translating call to printf to a call to the specialized version printf_va_1.
 [eva] Analyzing a complete application starting at main

src/plugins/variadic/translate.ml

@@ -89,8 +89,10 @@ let translate_variadics (file : file) =
     method! vglob glob =
       begin match glob with
         | GFunDecl(_, vi, _) when is_framac_builtin vi ->
-          Self.result ~level:2 ~current:true
-            "Variadic builtin %s left untransformed." vi.vname;
+          if Classify.classify env vi <> None then begin
+            Self.result ~level:2 ~current:true
+              "Variadic builtin %s left untransformed." vi.vname;
+          end;
           Cil.SkipChildren
 
         | GFunDecl(_, vi, _) ->