Rover: add method linking AP_Avoidance

APMrover2/system.cpp

@@ -337,3 +337,9 @@ bool Rover::is_boat() const
 {
     return ((enum frame_class)g2.frame_class.get() == FRAME_BOAT);
 }
+
+#include <AP_Avoidance/AP_Avoidance.h>
+#include <AP_ADSB/AP_ADSB.h>
+
+// dummy method to avoid linking AP_Avoidance
+AP_Avoidance *AP::ap_avoidance() { return nullptr; }