whois: add extra libs for libiconv

The main makefile doesn't check/do anything about linking properly, so
add -liconv to LIBS when it's used. Fixes:
http://autobuild.buildroot.net/results/7e5/7e551efacfa16b9e1915748a2f66fd1021d5bb49/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>