libinput: Suppress unhandled-case warning

When the wheel tilt source is present, gcc complains that we don't
handle all possible enumeration values. We already ensure this cannot
happen in its only caller (handle_pointer_axis), but gcc doesn't
recognise this. Give it a default value to quiet the warning.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>

libweston/libinput-device.c

@@ -191,6 +191,8 @@ normalize_scroll(struct libinput_event_pointer *pointer_event,
 		value = libinput_event_pointer_get_axis_value(pointer_event,
 							      axis);
 		break;
+	default:
+		assert(!"unhandled event source in normalize_scroll");
 	}
 
 	return value;