blacklist struct fields with reflection too

In the added test, the unexported field used to be garbled.

Reflection can only reach exported methods, exported fields, and
unexported fields. Exported methods and fields are currently never
garbled, so unexported fields was the only missing piece.

testdata/scripts/imports.txt

@@ -23,6 +23,7 @@ package main
 
 import (
 	"fmt"
+	"reflect"
 	_ "unsafe"
 
 	"test/main/imported"