parted: properly link with libiconv

parted uses libiconv but doesn't link with it. Add a patch to add it
to LIBADD of the library that uses it. iconv was already checked in
configure.ac, but only if i18n is enabled, so the iconv check is also
added unconditionally in configure.ac.

Also add an optional dependency on libiconv, so it is reproducible.

This was not detected in the autobuilders, since it only occurs when
libiconv exists (otherwise uClibc stubs will be used). libiconv
depends on !BR2_ENABLE_LOCALE and parted depends on BR2_USE_WCHAR. We
don't have such a configuration in the autobuilders.

Upstream status: sent to mailing list
http://lists.alioth.debian.org/pipermail/parted-devel/2017-November/005131.html



Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Cc: Sjoerd Venema <srg.venema@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>