Do not use require_relative with native extension

[Annih]

Replace require_relative "../llhttp_ext" by require "llhttp_ext" to load the native extension. This improves compatibility with Bundler and RubyGems, especially in environments using bundle install --standalone or other setups where the extension may not be located relative to the Ruby file.

Using require ensures the extension is loaded from the load path, following RubyGems best practices for native extensions.

Fix #33