added "is_reveiver" type alias in exec::reduce's receivers

Author: muellan

include/exec/reduce.hpp

@@ -35,6 +35,7 @@ namespace exec {
       };
 
       struct __t {
+        using is_receiver = void;
         using __id = __receiver;
         __data* __op_;
 

include/exec/static_thread_pool.hpp

@@ -526,6 +526,7 @@ namespace exec {
 
       template <class SenderId, class ReceiverId, class InitT, class RedOp>
       struct reduce_reciever {
+        using is_receiver = void;
         using Sender = stdexec::__t<SenderId>;
         using Receiver = stdexec::__t<ReceiverId>;