Fix C++20 modules include

Author: ldionne

libcxx/modules/std/stop_token.inc

@@ -9,7 +9,6 @@
 
 export namespace std {
 #ifndef _LIBCPP_HAS_NO_THREADS
-#  ifdef _LIBCPP_ENABLE_EXPERIMENTAL
   // [stoptoken], class stop_­token
   using std::stop_token;
 
@@ -22,6 +21,5 @@ export namespace std {
 
   // [stopcallback], class template stop_­callback
   using std::stop_callback;
-#  endif // _LIBCPP_ENABLE_EXPERIMENTAL
-#endif   // _LIBCPP_HAS_NO_THREADS
+#endif // _LIBCPP_HAS_NO_THREADS
 } // namespace std