add some encoding fields to Opcodes XML reader

internal/opcodesxml/opcodesxml_test.go

@@ -1,14 +1,12 @@
 package opcodesxml
 
 import (
-	"fmt"
 	"testing"
 )
 
 func TestReadFile(t *testing.T) {
-	is, err := ReadFile("testdata/x86_64.xml")
+	_, err := ReadFile("testdata/x86_64.xml")
 	if err != nil {
 		t.Fatal(err)
 	}
-	fmt.Printf("%#v\n", is)
 }