Add rexml dependency (#9007)

* Add rexml dependency

Since ruby 3.0.0, rexml is a bundled gem.
So it must be depend explicitly.

* Sort add_runtime_dependency

Author: znz

rb/selenium-webdriver.gemspec

@@ -32,6 +32,7 @@ Gem::Specification.new do |s|
   s.require_paths = ['lib']
 
   s.add_runtime_dependency 'childprocess', ['>= 0.5', '< 5.0']
+  s.add_runtime_dependency 'rexml'
   s.add_runtime_dependency 'rubyzip', ['>= 1.2.2']
   s.add_runtime_dependency 'websocket', ['~> 1.0']