AP_NavEKF: Update declination when we know our location

This ensures that when we start using GPS, that the EKF will be using the correct declination for that location
If declination is not known it defaults to zero

libraries/AP_NavEKF/AP_NavEKF.cpp

@@ -4163,6 +4163,8 @@ void NavEKF::readGpsData()
         } else if (goodToAlign){
             // Set the NE origin to the current GPS position
             setOrigin();
+            // Now we know the location we have an estimate for the magnetic field declination and adjust the earth field accordingly
+            alignMagStateDeclination();
             // Set the height of the NED origin to ‘height of baro height datum relative to GPS height datum'
             EKF_origin.alt = gpsloc.alt - hgtMea;
             // We are by definition at the origin at the instant of alignment so set NE position to zero