AP_NavEKF: make health() API const

libraries/AP_NavEKF/AP_NavEKF.cpp

@@ -68,7 +68,7 @@ NavEKF::NavEKF(const AP_AHRS *ahrs, AP_Baro &baro) :
     dtIMUAvgInv = 1.0f/dtIMUAvg;
 }
 
-bool NavEKF::healthy(void)
+bool NavEKF::healthy(void) const
 {
     if (!statesInitialised) {
         return false;

libraries/AP_NavEKF/AP_NavEKF.h

@@ -78,7 +78,7 @@ public:
     void UpdateFilter(void);
 
     // return true if the filter is healthy
-    bool healthy(void);
+    bool healthy(void) const;
 
     // fill in latitude, longitude and height of the reference point
     void getRefLLH(struct Location &loc) const;