BUILD: task: fix a build warning when threads are disabled

The __decl_hathreads() macro will leave a lone semi-colon making the end
of variables declarations, resulting in a warning if threads are disabled.
Let's simply swap it with the last variable. Thanks to Ilya Shipitsin for
reporting this issue.

No backport is needed.
diff --git a/src/task.c b/src/task.c
index 0ccc7af..d004a2c 100644
--- a/src/task.c
+++ b/src/task.c
@@ -161,8 +161,8 @@
 {
 	struct task *task;
 	struct eb32_node *eb;
-	__decl_hathreads(int key);
 	int ret = TICK_ETERNITY;
+	__decl_hathreads(int key);
 
 	while (1) {
   lookup_next_local: