I have created preference-headers.xml. I want to set header icon via `?attr`, but it doesn't show the icon.

```
<?xml version="1.0" encoding="utf-8"?>
<preference-headers xmlns:android="http://schemas.android.com/apk/res/android" >

<header
    android:fragment="com.armsoft.mtrade.activities.PreferencesActivity$PrefsAboutFragment"
    android:icon="?attr/menuIconAbout"
    android:title="@string/about" />

</preference-headers>

```