Update require statement for libxml_ruby 6.0.0 (#63)

Requiring libxml no longer works, but `libxml-ruby` does work on older
versions too.

Tested it locally with `bundle exec rake test XMLRPC_PARSER=libxml` with
libxml-ruby 6.0.0 and 5.0.6, I'm not sure if the CI includes this.

Co-authored-by: Herwin <herwinw@users.noreply.github.com>

Author: herwinw

lib/xmlrpc/parser.rb

@@ -595,7 +595,7 @@ def parse(str)
 
     class LibXMLStreamParser < AbstractStreamParser
       def initialize
-        require 'libxml'
+        require 'libxml-ruby'
         @parser_class = LibXMLStreamListener
       end