StatusBar: fix status bar clock category

During CyanogenMod merges

Error:

E/AndroidRuntime( 2586): FATAL EXCEPTION: main
E/AndroidRuntime( 2586): java.lang.RuntimeException:
    Unable to start activity ComponentInfo{com.android.settings/com.android.settings.SubSettings}:
    java.lang.ClassCastException: com.android.settings.cyanogenmod.SystemSettingCheckBoxPreference
    cannot be cast to android.preference.PreferenceCategory

Signed-off-by: Humberto Borba <humberos@gmail.com>