test: add development dependency to 'rexml' gem

* In order for the tests to work on Ruby 3
* see https://bugs.ruby-lang.org/issues/16485

Author: kenichiice

test-unit-runner-junitxml.gemspec

@@ -1,4 +1,3 @@
-
 lib = File.expand_path("../lib", __FILE__)
 $LOAD_PATH.unshift(lib) unless $LOAD_PATH.include?(lib)
 require "test/unit/runner/junitxml/version"
@@ -25,6 +24,7 @@ Gem::Specification.new do |spec|
 
   spec.add_development_dependency "bundler"
   spec.add_development_dependency "rake"
+  spec.add_development_dependency "rexml"
 
   spec.add_runtime_dependency "test-unit"
 end