PID: fix for HAL_SITL rename

libraries/PID/examples/pid/pid.pde

@@ -12,7 +12,7 @@
 #include <PID.h> // ArduPilot Mega RC Library
 
 #include <AP_HAL_AVR.h>
-#include <AP_HAL_AVR_SITL.h>
+#include <AP_HAL_SITL.h>
 #include <AP_HAL_Empty.h>
 
 const AP_HAL::HAL& hal = AP_HAL_BOARD_DRIVER;