whois: Fix encoding issues by linking iconv

whois.c hardcodes DENIC_PARAM_CHARSET " -C US-ASCII " if libiconv is not
defined, resulting in a lot of whois requests for .de domains to err out
saying "Error: 55000000013 Invalid charset for response", namely all
requests where the result contains non-ASCII chars like German umlauts.

Hence build and link with iconv support to prevent this from happening.

The `whois_LDADD+=-liconv` admittedly looks a bit nasty, but then again it works without patching the original Makefile.