thrdqueue: make thrdq_await_done only for unit tests

It is not used for anything else, so drop Curl_ and make it conditional
accordingly.

Closes #21499

tests/unit/unit3301.c

@@ -110,7 +110,7 @@ static CURLcode test_unit3301(const char *arg)
     fail_unless(!r, "queue-b send");
   }
 
-  r = Curl_thrdq_await_done(tqueue, 0);
+  r = thrdq_await_done(tqueue, 0);
   fail_unless(!r, "queue-b await done");
 
   nrecvd = 0;