MINOR: tasklets: Don't attempt to add a tasklet in the list twice.

Don't try to add a tasklet to the run queue if it's already in there, or we
might get an infinite loop.
diff --git a/include/proto/task.h b/include/proto/task.h
index d1371f3..8aceee5 100644
--- a/include/proto/task.h
+++ b/include/proto/task.h
@@ -205,6 +205,8 @@
 
 static inline void tasklet_wakeup(struct tasklet *tl)
 {
+	if (!LIST_ISEMPTY(&tl->list))
+		return;
 	LIST_ADDQ(&task_list[tid], &tl->list);
 	task_list_size[tid]++;
 	HA_ATOMIC_ADD(&tasks_run_queue, 1);