AP_NavEKF3: don't look at compass learning with GPS yaw

if using GPS yaw we don't want to do a yaw reset on completion of GPS
yaw, so just don't detect compass learning

libraries/AP_NavEKF3/AP_NavEKF3_Measurements.cpp

@@ -294,8 +294,12 @@ void NavEKF3_core::readMagData()
         return;
     }
 
-    if (compass.learn_offsets_enabled()) {
-        // while learning offsets keep all mag states reset
+    if (compass.learn_offsets_enabled() &&
+        frontend->sources.getYawSource() != AP_NavEKF_Source::SourceYaw::GPS_COMPASS_FALLBACK) {
+        /*
+          while learning offsets keep all mag states reset
+          note that compass learning is ignored with GPS with compass fallback
+        */
         InitialiseVariablesMag();
         wasLearningCompass_ms = imuSampleTime_ms;
     } else if (wasLearningCompass_ms != 0 && imuSampleTime_ms - wasLearningCompass_ms > 1000) {