AP_RangeFinder: fixed example build

libraries/AP_RangeFinder/examples/RFIND_test/RFIND_test.pde

@@ -29,6 +29,7 @@
 #include <AP_ADC_AnalogSource.h>
 #include <AP_Notify.h>
 #include <AP_Mission.h>
+#include <AP_Terrain.h>
 #include <AP_Scheduler.h>
 
 const AP_HAL::HAL& hal = AP_HAL_BOARD_DRIVER;