AP_Stats: make all semaphores recursive

the cost is very similar and this prevents an easy coding error which
can occur on less used code paths

libraries/AP_Stats/AP_Stats.h

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