devel/ruby-middleware: add PLIST

Add missing PLIST.


(taca)

@comment $NetBSD: PLIST,v 1.1 2020/01/18 16:11:29 taca Exp $
${GEM_HOME}/cache/${GEM_NAME}.gem
${GEM_LIBDIR}/.gitignore
${GEM_LIBDIR}/.travis.yml
${GEM_LIBDIR}/.yardopts
${GEM_LIBDIR}/CHANGELOG.md
${GEM_LIBDIR}/Gemfile
${GEM_LIBDIR}/LICENSE
${GEM_LIBDIR}/README.md
${GEM_LIBDIR}/Rakefile
${GEM_LIBDIR}/lib/middleware.rb
${GEM_LIBDIR}/lib/middleware/builder.rb
${GEM_LIBDIR}/lib/middleware/runner.rb
${GEM_LIBDIR}/lib/middleware/version.rb
${GEM_LIBDIR}/middleware.gemspec
${GEM_LIBDIR}/spec/middleware/builder_spec.rb
${GEM_LIBDIR}/spec/middleware/runner_spec.rb
${GEM_LIBDIR}/spec/setup.rb
${GEM_LIBDIR}/user_guide.md
${GEM_HOME}/specifications/${GEM_NAME}.gemspec