net-p2p/*arr: Switch back to converters/libiconv dependency

In PR 274169 all *arr ports were switched to use 'USES+=libiconv' instead of a
hard dependency on the 'converters/libiconv' port. Turns out, this is
incorrect.

Specifically 'libMonoPosixHelper' (part of .NET) is compiled against the port,
and will break when the port is not installed (incompatible with the base
version of libiconv)

This patch moves all *arr ports back to RUN_DEPENDS on converters/libiconv

PR: 274532
Reported by: michiel@vanbaak.eu (maintainer)