Require Rake before referencing Rake::DSL

This file references `Rake::DSL`, and may be required in isolation, and so it
should `require "rake"` before using it.

Author: sambostock

lib/graphql/rake_task.rb

@@ -1,5 +1,6 @@
 # frozen_string_literal: true
 require "fileutils"
+require "rake"
 require "graphql/rake_task/validate"
 
 module GraphQL