[ruby/rdoc] Fix failing test by adding gettext as a development dependency

The failing test was:

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rdoc/test/rdoc/test_rdoc_i18n_locale.rb:35:in `rescue in test_load_existent_po'
Omission: gettext gem is not found [test_load_existent_po(TestRDocI18nLocale)]
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https://github.com/ruby/rdoc/commit/e5f85839e1

lib/rdoc/rdoc.gemspec

@@ -243,4 +243,6 @@ RDoc includes the +rdoc+ and +ri+ tools for generating and displaying documentat
 
   s.required_ruby_version = Gem::Requirement.new(">= 2.4.0")
   s.required_rubygems_version = Gem::Requirement.new(">= 2.2")
+
+  s.add_development_dependency("gettext")
 end