Add approve workflow to document library

     1-   Go to schema.xml in document library

2- Add  ModeratedList="TRUE"  in line of list

Ex:

<List xmlns:ows="Microsoft SharePoint" EnableContentTypes="TRUE" Title="Articles" Direction="$Resources:Direction;" Url=Lists/Articles" BaseType="1" xmlns="http://schemas.microsoft.com/sharepoint/" ModeratedList="TRUE" DraftVersionVisibility="2">