std::once_flag was forward declared with _LIBCPP_TYPE_VIS decoration, and the defined with _LIBCPP_TYPE_VIS_ONLY decoration. Make them match

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@214219 91177308-0d34-0410-b5e6-96231b3b80d8