iconv: Fix "make make-ref"

The purpose of this command is to "refresh" the source reference
files generated with GNU libiconv, located in tools/test/iconv/ref/.
Previously it would generate copies somewhere in OBJDIR, which
we don't use.

Reviewed By: allanjude
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D31820