async-thread: avoid closing eventfd twice

When employing eventfd for socketpair, there is only one file
descriptor. Closing that fd twice might result in fd corruption.
Thus, we should avoid closing the eventfd twice, following the
pattern in lib/multi.c.

Fixes #15725
Closes #15727
Reported-by: Christian Heusel

lib/asyn-thread.c

@@ -195,9 +195,11 @@ void destroy_thread_sync_data(struct thread_sync_data *tsd)
    * close one end of the socket pair (may be done in resolver thread);
    * the other end (for reading) is always closed in the parent thread.
    */
+#ifndef USE_EVENTFD
   if(tsd->sock_pair[1] != CURL_SOCKET_BAD) {
     wakeup_close(tsd->sock_pair[1]);
   }
+#endif
 #endif
   memset(tsd, 0, sizeof(*tsd));
 }