ruby.h: fix typo

* include/ruby/ruby.h (ruby_check_sizeof_long_long): fix typo of
  availablity macro.  [ruby-dev:47341]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40739 b2dd03c8-39d4-4d8f-98ff-823fe69b080e