MINOR: task: Fix a compiler warning by adding a cast.

When calling HA_ATOMIC_CAS with a pointer as the target, the compiler
expects a pointer as the new value, so give it one by casting 0x1 to
(void *).
diff --git a/include/proto/task.h b/include/proto/task.h
index 760b368..0c2c5f2 100644
--- a/include/proto/task.h
+++ b/include/proto/task.h
@@ -218,7 +218,7 @@
 	 * another runqueue. We set leaf_p to 0x1 to indicate that the node is
 	 * not in a tree but that it's in the tasklet list. See task_in_rq().
 	 */
-	if (unlikely(!HA_ATOMIC_CAS(&t->rq.node.leaf_p, &expected, 0x1)))
+	if (unlikely(!HA_ATOMIC_CAS(&t->rq.node.leaf_p, &expected, (void *)0x1)))
 		return;
 	HA_ATOMIC_ADD(&tasks_run_queue, 1);
 	task_list_size[tid]++;