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/dict.h b/include/types/dict.h
index 9807816..def6355 100644
--- a/include/types/dict.h
+++ b/include/types/dict.h
@@ -13,7 +13,7 @@
 struct dict {
 	const char *name;
 	struct eb_root values;
-	__decl_hathreads(HA_RWLOCK_T rwlock);
+	__decl_thread(HA_RWLOCK_T rwlock);
 };
 
 #endif /* _TYPES_DICT_H */