AC_PrecLand: fix backend compile error

libraries/AC_PrecLand/AC_PrecLand_Backend.h

@@ -27,7 +27,7 @@ public:
     //  returns true if new data available
     virtual bool update() = 0;
     // what frame of reference is our sensor reporting in?
-    virtual MAV_FRAME get_frame_of_reference();
+    virtual MAV_FRAME get_frame_of_reference() = 0;
 
     // get_angle_to_target - returns angles (in radians) to target
     //  returns true if angles are available, false if not (i.e. no target)