Fix broken hscolour.css symlink in tarball

The file apidoc/Ganeti/HTools/hscolour.css was a broken symlink in the
tarball. This was happening because `tar --transform` was being allowed to
act in symlink targets.

Pass the flag "S" to tar --transform, to keep symlink targets intact (and
hence valid).

Signed-off-by: Adeodato Simo <dato@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>