avdevice/xcbgrab: set avclass category

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavdevice/xcbgrab.c

@@ -92,6 +92,7 @@ static const AVClass xcbgrab_class = {
     .item_name  = av_default_item_name,
     .option     = options,
     .version    = LIBAVUTIL_VERSION_INT,
+    .category   = AV_CLASS_CATEGORY_DEVICE_VIDEO_INPUT,
 };
 
 static int xcbgrab_reposition(AVFormatContext *s,