Include gemspec in ExtensionTask for native gem tasks

[mgmarlow]

What was the end-user or developer problem that led to this PR?

Original issue stated in oxidize-rb/rb-sys#249.

New gems created with --ext=rust do not include the native build tasks from rake-compiler.

What is your fix for the problem, implemented in this PR?

Pass the gemspec into the second argument to RbSys::ExtensionTask so the native gem tasks are available for Rake. Users can now run bundle exec rake native gem after generating a new Rust extension.

Make sure the following tasks are checked

• Describe the problem / feature
• Write tests for features and bug fixes
• Write code to solve the problem
• Make sure you follow the current code style and write meaningful commit messages without tags

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