libavdevice/gdigrab: fix ffmpeg -devices doesn't show gdigrab

missed the category AV_CLASS_CATEGORY_DEVICE_VIDEO_INPUT lead to
ffmpeg -devices doesn't show gdigrab as a input device

FIx #7848

Found-by: dangibson
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Jun Zhao <barryjzhao@tencent.com>

libavdevice/gdigrab.c

@@ -647,6 +647,7 @@ static const AVClass gdigrab_class = {
     .item_name  = av_default_item_name,
     .option     = options,
     .version    = LIBAVUTIL_VERSION_INT,
+    .category   = AV_CLASS_CATEGORY_DEVICE_VIDEO_INPUT,
 };
 
 /** gdi grabber device demuxer declaration */