AP_NavEKF3: fix using_external_yaw when using external nav

the first if would always return true meaning the method would always return false when using external nav
thanks to David Sastre for finding this!

libraries/AP_NavEKF3/AP_NavEKF3_Control.cpp

@@ -537,16 +537,16 @@ bool NavEKF3_core::use_compass(void) const
 // are we using a yaw source other than the magnetomer?
 bool NavEKF3_core::using_external_yaw(void) const
 {
-#if EK3_FEATURE_EXTERNAL_NAV
     const AP_NavEKF_Source::SourceYaw yaw_source = frontend->sources.getYawSource();
-    if (yaw_source == AP_NavEKF_Source::SourceYaw::GPS || yaw_source == AP_NavEKF_Source::SourceYaw::GPS_COMPASS_FALLBACK ||
-        yaw_source == AP_NavEKF_Source::SourceYaw::GSF || !use_compass()) {
-        return imuSampleTime_ms - last_gps_yaw_fusion_ms < 5000 || imuSampleTime_ms - lastSynthYawTime_ms < 5000;
-    }
+#if EK3_FEATURE_EXTERNAL_NAV
     if (yaw_source == AP_NavEKF_Source::SourceYaw::EXTNAV) {
         return ((imuSampleTime_ms - last_extnav_yaw_fusion_ms < 5000) || (imuSampleTime_ms - lastSynthYawTime_ms < 5000));
     }
 #endif
+    if (yaw_source == AP_NavEKF_Source::SourceYaw::GPS || yaw_source == AP_NavEKF_Source::SourceYaw::GPS_COMPASS_FALLBACK ||
+        yaw_source == AP_NavEKF_Source::SourceYaw::GSF || !use_compass()) {
+        return imuSampleTime_ms - last_gps_yaw_fusion_ms < 5000 || imuSampleTime_ms - lastSynthYawTime_ms < 5000;
+    }
     return false;
 }