racc/cparse: private class

* ext/racc/cparse/cparse.c (Init_cparse): Racc::CparseParams is a
  private class, undefine allocate and initialize methods.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47739 b2dd03c8-39d4-4d8f-98ff-823fe69b080e