Files of type Symlink are not written correctly

[antoineco]

The library is able to deserialize Header structs from Symlink files: https://github.com/cavaliergopher/cpio/blob/b4d3577d12deb76a6d52f184acd341f875574785/svr4.go#L90-L101

However, it completely ignores the Linkname attribute while writing the header, resulting in zero-sized files without target.

Note that using writer.Write(target) as a workaround isn't an option because the method isn't supported for files other than Regular: https://github.com/cavaliergopher/cpio/blob/b4d3577d12deb76a6d52f184acd341f875574785/writer.go#L103-L105