fix: add emqx_dashboard as a dependency of emqx_machine

```
===> Compiling emqx_machine
===> Compiling src/user_default.erl failed
src/user_default.erl:{24,14}: can't find include lib "emqx_dashboard/include/emqx_dashboard.hrl"; Make sure emqx_dashboard is in your app file's 'applications' list

==> emqx_mix
** (Mix) Could not compile dependency :emqx_machine, "/github/home/.mix/elixir/1-14/rebar3 bare compile --paths /__w/emqx/emqx/_build/emqx/lib/*/ebin" command failed. Errors may have been logged above. You can recompile this dependency with "mix deps.compile emqx_machine", update it with "mix deps.update emqx_machine" or clean it with "mix deps.clean emqx_machine"
make: *** [Makefile:276: emqx-elixir] Error 1
```

apps/emqx_machine/rebar.config

@@ -2,6 +2,7 @@
 
 {deps, [
     {emqx, {path, "../emqx"}},
+    {emqx_dashboard, {path, "../emqx_dashboard"}},
     {emqx_utils, {path, "../emqx_utils"}}
 ]}.