rexml is bundled gem in Ruby 3 (#899)

google-api-client.gemspec

@@ -30,6 +30,7 @@ Gem::Specification.new do |spec|
   spec.add_runtime_dependency 'signet', '~> 0.12'
   spec.add_runtime_dependency 'googleauth', '~> 0.9'
   spec.add_runtime_dependency 'httpclient', '>= 2.8.1', '< 3.0'
+  spec.add_runtime_dependency 'rexml'
   spec.add_development_dependency 'thor', '~> 0.19'
   spec.add_development_dependency 'activesupport', '>= 4.2', '< 5.1'
 end