Fix a method redefinition warning in C parser

Ref: #728
Ref: ruby/ruby#12569

Author: byroot

lib/json/ext.rb

@@ -11,6 +11,7 @@ class << self
         def parse(...)
           new(...).parse
         end
+        alias_method :parse, :parse # Allow redefinition by extensions
       end
 
       def initialize(source, opts = nil)