* ruby_atomic.h: fixed merge mistake of r44946.  reported by ngoto at
  [ruby-dev:47980] [Backport #9530]


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_1_9_3@45036 b2dd03c8-39d4-4d8f-98ff-823fe69b080e