x86, build: copy ldlinux.c32 to image.iso

commit 9c77679c upstream.

For newer versions of Syslinux, we need ldlinux.c32 in addition to
isolinux.bin to reside on the boot disk, so if the latter is found,
copy it, too, to the isoimage tree.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Cc: Linux Stable Tree <stable@vger.kernel.org>
Signed-off-by: Willy Tarreau <w@1wt.eu>