[Hotfix] Master compilation error on MacOS.

[simon-mo]

Ray won't compile on mac:

Use --sandbox_debug to see verbose messages from the sandbox
src/ray/core_worker/reference_count.cc:1013:19: error: loop variable 'callback' of type 'const std::__1::function<void (const absl::lts_2019_08_08::flat_hash_set<ray::NodeID, absl::lts_2019_08_08::hash_internal::Hash<ray::NodeID>, std::__1::equal_to<ray::NodeID>, std::__1::allocator<ray::NodeID> > &, long long, long long)>' creates a copy from type 'const std::__1::function<void (const absl::lts_2019_08_08::flat_hash_set<ray::NodeID, absl::lts_2019_08_08::hash_internal::Hash<ray::NodeID>, std::__1::equal_to<ray::NodeID>, std::__1::allocator<ray::NodeID> > &,
  0 [Hotfix] Master compile and dask test
 long long, long long)>' [-Werror,-Wrange-loop-analysis]
  for (const auto callback : callbacks) {
                  ^
src/ray/core_worker/reference_count.cc:1013:8: note: use reference type 'const std::__1::function<void (const absl::lts_2019_08_08::flat_hash_set<ray::NodeID, absl::lts_2019_08_08::hash_internal::Hash<ray::NodeID>, std::__1::equal_to<ray::NodeID>, std::__1::allocator<ray::NodeID> > &, long long, long long)> &' to prevent copying
  for (const auto callback : callbacks) {
       ^~~~~~~~~~~~~~~~~~~~~
                  &
1 error generated.
Target //:ray_pkg failed to build
INFO: Elapsed time: 11.263s, Critical Path: 10.72s
INFO: 2 processes: 2 darwin-sandbox.