add tud_hid_n_gamepad_report() helper for gamepad report

- Add gamepad to hid_composite example. Though it needs a bit of extra
work but it will come later as separated PR.

examples/device/hid_composite/src/usb_descriptors.h

@@ -30,6 +30,7 @@ enum
   REPORT_ID_KEYBOARD = 1,
   REPORT_ID_MOUSE,
   REPORT_ID_CONSUMER_CONTROL,
+  REPORT_ID_GAMEPAD
 };
 
 #endif /* USB_DESCRIPTORS_H_ */