Sub: add methods to avoid linking AP_Avoidance

ArduSub/system.cpp

@@ -304,7 +304,12 @@ bool Sub::should_log(uint32_t mask)
 }
 
 #include <AP_AdvancedFailsafe/AP_AdvancedFailsafe.h>
+#include <AP_Avoidance/AP_Avoidance.h>
+#include <AP_ADSB/AP_ADSB.h>
 
 // dummy method to avoid linking AFS
 bool AP_AdvancedFailsafe::gcs_terminate(bool should_terminate, const char *reason) { return false; }
 AP_AdvancedFailsafe *AP::advancedfailsafe() { return nullptr; }
+
+// dummy method to avoid linking AP_Avoidance
+AP_Avoidance *AP::ap_avoidance() { return nullptr; }