include/ruby/ruby.h: hide Symbol internals

This is too new to be a compatibility problem.

* include/ruby/ruby.h: Hide Symbol internals.
  (struct RSymbol): moved to internal.h
  (RSYMBOL): ditto

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46219 b2dd03c8-39d4-4d8f-98ff-823fe69b080e