CLEANUP: thread: rename __decl_hathreads() to __decl_thread()

I can never figure whether it takes an "s" or not, and in the end it's
better if it matches the file's naming, so let's call it "__decl_thread".
diff --git a/include/proto/server.h b/include/proto/server.h
index 4c4b4bf..e184310 100644
--- a/include/proto/server.h
+++ b/include/proto/server.h
@@ -37,7 +37,7 @@
 #include <proto/freq_ctr.h>
 
 
-__decl_hathreads(extern HA_SPINLOCK_T idle_conn_srv_lock);
+__decl_thread(extern HA_SPINLOCK_T idle_conn_srv_lock);
 extern struct eb_root idle_conn_srv;
 extern struct task *idle_conn_task;
 extern struct task *idle_conn_cleanup[MAX_THREADS];