Don't limit cursor headers to wc.headers

I'm about to wrap Freetype, where the recommended way of including is the following:

```
#include <ft2build.h>
#include FT_FREETYPE_H
```

I therefore can't add Freetype's headers directly to wc.headers. But as I understood it correctly that's what the function `header_wrapped` is for, which looks like this for me:

```
function should_wrap(hdr::ASCIIString, name::ASCIIString)
    return beginswith(dirname(name), FREETYPE_INCLUDE)
end
```

But because of the check for `cursor_hdr in wc.headers` I can't let wrap_c include the Freetype headers.