Correctly handle LIBUSB_TRANSFER_OVERFLOW in libusb_control_transfer()

sync.c: In function `libusb_control_transfer':
sync.c:122: warning: enumeration value `LIBUSB_TRANSFER_OVERFLOW' not
handled in switch

Fixes #120.
diff --git a/libusb/sync.c b/libusb/sync.c
index d50413b..8eed47b 100644
--- a/libusb/sync.c
+++ b/libusb/sync.c
@@ -132,6 +132,9 @@
 	case LIBUSB_TRANSFER_NO_DEVICE:
 		r = LIBUSB_ERROR_NO_DEVICE;
 		break;
+	case LIBUSB_TRANSFER_OVERFLOW:
+		r = LIBUSB_ERROR_OVERFLOW;
+		break;
 	default:
 		usbi_warn(HANDLE_CTX(dev_handle),
 			"unrecognised status code %d", transfer->status);