Fix compiler warnings by redordering class members.

Change-Id: I780238c7695a326c088917cdd5b389d279d3d2c7
diff --git a/lib/ethosu_monitor/include/ethosu_monitor.hpp b/lib/ethosu_monitor/include/ethosu_monitor.hpp
index fa5a63f..c2d0908 100644
--- a/lib/ethosu_monitor/include/ethosu_monitor.hpp
+++ b/lib/ethosu_monitor/include/ethosu_monitor.hpp
@@ -60,8 +60,8 @@
 private:
     ethosu_pmu_event_type ethosuEventIds[ETHOSU_PMU_NCOUNTERS];
     size_t numEvents;
-    Backend backend;
     std::vector<int32_t> eventRecordIds;
+    Backend backend;
 };
 
 #endif