kmsgrab: add category for kmsgrab

Makes kmsgrab visible in "ffmpeg -devices".
Signed-off-by: Jun Zhao <mypopydev@gmail.com>
Signed-off-by: Mark Thompson <sw@jkqxz.net>

libavdevice/kmsgrab.c

@@ -451,6 +451,7 @@ static const AVClass kmsgrab_class = {
     .item_name  = av_default_item_name,
     .option     = options,
     .version    = LIBAVUTIL_VERSION_INT,
+    .category   = AV_CLASS_CATEGORY_DEVICE_VIDEO_INPUT,
 };
 
 AVInputFormat ff_kmsgrab_demuxer = {