fix dynamic linking of stats on macOS

Summary:
D14531266 defaulted static linking on macOS because of a link error in
folly/stats. A better fix for the issue is instantiating the
subclasses of BufferedStat, so add those.

Reviewed By: andrewjcg

Differential Revision: D14767345

fbshipit-source-id: eaa597dec468264595547f338348d7d74b49dd69

folly/stats/detail/BufferedStatTDigest.cpp

@@ -21,6 +21,8 @@ namespace folly {
 namespace detail {
 
 template class BufferedStat<TDigest, std::chrono::steady_clock>;
+template class BufferedDigest<TDigest, std::chrono::steady_clock>;
+template class BufferedSlidingWindow<TDigest, std::chrono::steady_clock>;
 
 } // namespace detail
 } // namespace folly

folly/stats/detail/BufferedStatTDigest.h

@@ -23,6 +23,8 @@ namespace folly {
 namespace detail {
 
 extern template class BufferedStat<TDigest, std::chrono::steady_clock>;
+extern template class BufferedDigest<TDigest, std::chrono::steady_clock>;
+extern template class BufferedSlidingWindow<TDigest, std::chrono::steady_clock>;
 
 } // namespace detail
 } // namespace folly