AC_PID: update example sketch to test AC_HELI_PID

libraries/AC_PID/examples/AC_PID_test/AC_PID_test.pde

@@ -10,6 +10,7 @@
 #include <AP_Math.h>
 #include <AP_Param.h>
 #include <AC_PID.h>
+#include <AC_HELI_PID.h>
 
 const AP_HAL::HAL& hal = AP_HAL_BOARD_DRIVER;
 
@@ -32,6 +33,7 @@ void loop()
 {
     // setup (unfortunately must be done here as we cannot create a global AC_PID object)
     AC_PID pid(TEST_P, TEST_I, TEST_D, TEST_IMAX * 100);
+    AC_HELI_PID heli_pid(TEST_P, TEST_I, TEST_D, TEST_IMAX * 100);
     uint16_t radio_in;
     uint16_t radio_trim;
     int error;