Mix: Fix ejabberdctl iexlive problem locating iex in an OTP release

The problem appeared when:
  ./configure --with-rebar=mix
  make dev
  _build/dev/rel/ejabberd/bin/ejabberdctl iexlive

mix.exs

@@ -238,6 +238,8 @@ defmodule Ejabberd.MixProject do
       config_dir: config(:config_dir),
       logs_dir: config(:logs_dir),
       spool_dir: config(:spool_dir),
+      vsn: config(:vsn),
+      iex: config(:iex),
       erl: config(:erl),
       epmd: config(:epmd),
       bindir: Path.join([config(:release_dir), "releases", version()]),

vars.config.in

@@ -53,6 +53,7 @@
 {sysconfdir, "{{release_dir}}/etc"}.
 {erts_dir, "{{release_dir}}/erts-${ERTS_VSN#erts-}"}.
 {installuser, "@INSTALLUSER@"}.
+{iex, "{{release_dir}}/releases/{{vsn}}/iex"}.
 {erl, "{{erts_dir}}/bin/erl"}.
 {epmd, "{{erts_dir}}/bin/epmd"}.
 {localstatedir, "{{release_dir}}/var"}.