Add rexml as an implicit thermite dependency

Author: antstorm

temporalio.gemspec

@@ -22,6 +22,7 @@ Gem::Specification.new do |spec|
   spec.required_ruby_version = '>= 2.7.0'
 
   spec.add_dependency 'google-protobuf', '~> 3.21.1' # Protobuf
+  spec.add_dependency 'rexml', '~> 3.2.5' # Implicitly required by thermite
   spec.add_dependency 'rutie', '~> 0.0.4' # Rust bindings
   spec.add_dependency 'thermite', '~> 0.13.0' # For compiling Rust ext