Use require_relative for yarp/ffi

Author: eregon

lib/yarp.rb

@@ -515,5 +515,5 @@ def self.parse_serialize_file(filepath)
 if RUBY_ENGINE == "ruby" and !ENV["YARP_FFI_BACKEND"]
   require "yarp/yarp"
 else
-  require "yarp/ffi"
+  require_relative "yarp/ffi"
 end