module directives: move `@derive` to before `defstruct`

[novaugust]

fixes compiler warning

Example Input

iex(1)> defmodule Foo do
...(1)>   defstruct [:bar]
...(1)>   @derive {Inspect, only: [:bar]}
...(1)> end
warning: warning: module attribute @derive was set after defstruct, all @derive calls must come before defstruct
└─ iex:1: Foo (module)

[novaugust]

cc @milmazz :)