package/whois: add host-pkgconf mandatory dependency

package/whois: add host-pkgconf mandatory dependency

pkg-config is used to check if libidn2 is available since version 5.2.18
and
https://github.com/rfc1036/whois/commit/e20b8bbab3687aa26294bc3c443b4a43292b7636

This can lead to a build failure if a broken libidn2.pc is found on host
so add a mandatory host-pkgconf dependency and drop our manual handling
of HAVE_LIBIDN2 option (which does not exist anymore)

Fixes:
 - http://autobuild.buildroot.org/results/6cd0dd779d72b7162943f0f7dbb4e7d258b463eb

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>