Effort to reduce the number of exported symbols

llvm-svn: 122057
diff --git a/libcxx/include/thread b/libcxx/include/thread
index abb56f4..8b3edb7 100644
--- a/libcxx/include/thread
+++ b/libcxx/include/thread
@@ -297,7 +297,7 @@
 
 class __assoc_sub_state;
 
-class __thread_struct_imp;
+class _LIBCPP_HIDDEN __thread_struct_imp;
 
 class __thread_struct
 {