AP_Stats: Add missing const in member functions

Signed-off-by: Patrick José Pereira <patrickelectric@gmail.com>

libraries/AP_Stats/AP_Stats.h

@@ -32,7 +32,7 @@ public:
     void set_flying(bool b);
 
     // accessor for is_flying
-    bool get_is_flying(void) {
+    bool get_is_flying(void) const {
         return _flying_ms != 0;
     }