Changing aos_sync and y2019 constants from once_ to once

Change-Id: Ib86f936087c0e3b254ff8ddf801149722c5e6549
diff --git a/y2019/constants.cc b/y2019/constants.cc
index 5466ce2..d5b0c09 100644
--- a/y2019/constants.cc
+++ b/y2019/constants.cc
@@ -239,9 +239,9 @@
 }  // namespace
 
 const Values &GetValues() {
-  static absl::once_flag once_;
+  static absl::once_flag once;
   static const Values* result;
-  absl::call_once(once_, DoGetValues, &result);
+  absl::call_once(once, DoGetValues, &result);
   return *result;
 }