liblog: add android_log_timestamp() private function

Change-Id: Iefbea3b1be2f97cfdeb35e5330e5495e0337215b
diff --git a/include/log/logger.h b/include/log/logger.h
index f030dab..c795253 100644
--- a/include/log/logger.h
+++ b/include/log/logger.h
@@ -183,6 +183,8 @@
                                              pid_t pid);
 #define android_logger_list_close android_logger_list_free
 
+char android_log_timestamp();
+
 /*
  * log_id_t helpers
  */