cont.c: fix build when FIBER_USE_NATIVE is 0
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47966 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
cont.c: fix build when FIBER_USE_NATIVE is 0
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47966 b2dd03c8-39d4-4d8f-98ff-823fe69b080e