AP_Stats: added semaphore to flttime update

fixes #12948

libraries/AP_Stats/AP_Stats.cpp

@@ -76,6 +76,7 @@ void AP_Stats::flush()
 void AP_Stats::update_flighttime()
 {
     if (_flying_ms) {
+        WITH_SEMAPHORE(sem);
         const uint32_t now = AP_HAL::millis();
         const uint32_t delta = (now - _flying_ms)/1000;
         flttime += delta;
@@ -93,6 +94,7 @@ void AP_Stats::update_runtime()
 
 void AP_Stats::update()
 {
+    WITH_SEMAPHORE(sem);
     const uint32_t now_ms = AP_HAL::millis();
     if (now_ms -  last_flush_ms > flush_interval_ms) {
         update_flighttime();

libraries/AP_Stats/AP_Stats.h

@@ -67,7 +67,7 @@ private:
 
     void update_flighttime();
     void update_runtime();
-
+    HAL_Semaphore_Recursive sem;
 };
 
 namespace AP {