The IDNA changes for SMTPUTF8 support introduced a bug for systems
without iconv.  For those systems, the local<->intl functions would
return an error due to the charset conversion failing.

Change mutt_idna.c back to being conditionally compiled, but this time
based on HAVE_ICONV.  If there is no iconv, stub out the functions in
mutt_idna.h.