Build x64-mingw-ucrt gem

RubyInstaller 3.1 has switched C-Runtime to UCRT so that RUBY_PLATFORM
has been changed to `x64-mingw-ucrt`:

https://rubyinstaller.org/2021/12/31/rubyinstaller-3.1.0-1-released.html

Signed-off-by: Takuro Ashie <ashie@clear-code.com>

Rakefile

@@ -14,7 +14,7 @@ namespace :build do
   desc 'Build gems for all platforms'
   task :all do
     Bundler.with_clean_env do
-      %w[ruby x86-mingw32 x64-mingw32].each do |name|
+      %w[ruby x86-mingw32 x64-mingw32 x64-mingw-ucrt].each do |name|
         ENV['GEM_BUILD_FAKE_PLATFORM'] = name
         Rake::Task["build"].execute
       end