Fix build for IDN when not using BINDTOOLS by correcting the RUN_DEPENDS:

Remove: idn2:dns/idnkit2
Add: idn2:dns/libidn2

I believe this is a case of incorrect RUN_DEPENDS.

The problem does not appear when using BINDTOOLS because that option pulls
in libidn2 via dns/bind-tools

Reported by: Russian <russian@russerver.org>
MFH: 2019Q3