ruby.h: RB_UNUSED_VAR from UNINITIALIZED_VAR

* include/ruby/ruby.h (RB_UNUSED_VAR): move code from
  UNINITIALIZED_VAR() in vm_core.h.
* vm_core.h (UNINITIALIZED_VAR): use RB_UNUSED_VAR().

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39861 b2dd03c8-39d4-4d8f-98ff-823fe69b080e