Effort to reduce the number of exported symbols

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@122057 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/thread b/include/thread
index abb56f4..8b3edb7 100644
--- a/include/thread
+++ b/include/thread
@@ -297,7 +297,7 @@
 
 class __assoc_sub_state;
 
-class __thread_struct_imp;
+class _LIBCPP_HIDDEN __thread_struct_imp;
 
 class __thread_struct
 {