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/types/vars.h b/include/types/vars.h
index e7d71d3..62b944b 100644
--- a/include/types/vars.h
+++ b/include/types/vars.h
@@ -19,7 +19,7 @@
 	struct list head;
 	enum vars_scope scope;
 	unsigned int size;
-	__decl_hathreads(HA_RWLOCK_T rwlock);
+	__decl_thread(HA_RWLOCK_T rwlock);
 };
 
 /* This struct describes a variable. */