zxing: don't select libiconv

The zxing package currently selects libiconv when locale support is not
available in the toolchain.  This patch stops the zxing package selecting
libiconv.  The zxing package now uses uses libiconv if it's available
otherwise iconv support is disabled in zxing.

Signed-off-by: Martin Bark <martin@barkynet.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>