`-g doc-includes` doesn't generate `<include>` for static fields

[source](https://github.com/erincatto/box3d/blob/23861418d877fdd97990f35645e845e44a7eb9ec/include/box3d/id.h#L92)
```c
static const b3WorldId b3_nullWorldId = B3_NULL_ID;
```
generates
```cs
    [NativeTypeName("const b3WorldId")]
    public static readonly b3WorldId b3_nullWorldId = new b3WorldId
    {
        index1 = 0,
    };
```
without the `<include>`.