AP_NavEKF2: Add missing const in member functions

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

libraries/AP_NavEKF2/AP_NavEKF2_Outputs.cpp

@@ -572,7 +572,7 @@ uint8_t NavEKF2_core::getFramesSincePredict(void) const
 }
 
 // return true when external nav data is also being used as a yaw observation
-bool NavEKF2_core::isExtNavUsedForYaw()
+bool NavEKF2_core::isExtNavUsedForYaw() const
 {
     return extNavUsedForYaw;
 }

libraries/AP_NavEKF2/AP_NavEKF2_core.h

@@ -309,7 +309,7 @@ public:
     void writeExtNavVelData(const Vector3f &vel, float err, uint32_t timeStamp_ms, uint16_t delay_ms);
 
     // return true when external nav data is also being used as a yaw observation
-    bool isExtNavUsedForYaw(void);
+    bool isExtNavUsedForYaw(void) const;
 
     // Writes the default equivalent airspeed in m/s to be used in forward flight if a measured airspeed is required and not available.
     void writeDefaultAirSpeed(float airspeed);