fix: correct secure_compare indentation to remove Ruby 4 warning

[masterkain]

Summary

• fix the indentation of secure_compare in lib/omniauth/strategies/oauth2.rb
• remove Ruby 4 warning: mismatched indentations at 'end' with 'def'

Reproduction

ruby -w -e "gem 'omniauth-oauth2','1.9.0'; require 'omniauth-oauth2'"

Before this change, Ruby 4 emits:

warning: mismatched indentations at 'end' with 'def'

Verification

ruby -w -c lib/omniauth/strategies/oauth2.rb
bundle exec rspec spec/omniauth/strategies/oauth2_spec.rb