[ruby/singleton] Only use RactorLocalSingleton if Ractor is defined

https://github.com/ruby/singleton/commit/f684d36a47

lib/singleton.rb

@@ -196,6 +196,7 @@ module Singleton
   #  Returns the singleton instance.
 end
 
+if defined?(Ractor)
   module RactorLocalSingleton
     include Singleton::SingletonInstanceMethods
 
@@ -236,3 +237,4 @@ module RactorLocalSingleton
 
     extend Singleton::SingletonClassProperties
   end
+end