Fix xref exclude deprecation warning

[fhunleth]

This fixes the following warning with Elixir 1.20:

warning: "xref: [exclude: ...]" in your mix.exs file is deprecated, instead use: "elixirc_options: [no_warn_undefined: ...]"
  (mix 1.20.2) lib/mix/tasks/compile.elixir.ex:243: Mix.Tasks.Compile.Elixir.xref_exclude_opts/2
  (mix 1.20.2) lib/mix/tasks/compile.elixir.ex:142: Mix.Tasks.Compile.Elixir.run/1
  (mix 1.20.2) lib/mix/task.ex:502: anonymous fn/3 in Mix.Task.run_task/5
  (mix 1.20.2) lib/mix/task.compiler.ex:299: Mix.Task.Compiler.run_compiler/2
  (mix 1.20.2) lib/mix/task.compiler.ex:287: Mix.Task.Compiler.run/4
  (mix 1.20.2) lib/mix/tasks/compile.all.ex:75: Mix.Tasks.Compile.All.do_run/2

[josevalim]

We should probably move this to that module then?

@compile {:no_warn_undefined, [:wx, :wx_object, :wxFrame, :wxWebView]}

Instead of a global? 🤔

[fhunleth]

You caught me trying to get away with the minimal change to reduce the warnings I get when building Nerves Livebook. :)

PR updated with the local change.

[josevalim]

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