`-g doc-includes` doesn't generate <include> for `-g macro-bindings`

A value-like macro binding ([source](https://github.com/erincatto/box3d/blob/23861418d877fdd97990f35645e845e44a7eb9ec/include/box3d/math_functions.h#L20-L21)):
```c
#define B3_PI 3.14159265359f
```
generates
```cs
    [NativeTypeName("#define B3_PI 3.14159265359f")]
    public const float B3_PI = 3.14159265359f;
```
without the `<include>` supposedly generated by `-g doc-includes`