lavfi: fix can't dispaly "slice" sub-option in "ffmpeg -h full"

 fix can't dispaly "slice" sub-option in "ffmpeg -h full" for
 AVFilter options.
Signed-off-by: Jun Zhao <mypopydev@gmail.com>

libavfilter/avfilter.c

@@ -628,7 +628,7 @@ static const AVClass *filter_child_class_next(const AVClass *prev)
 static const AVOption avfilter_options[] = {
     { "thread_type", "Allowed thread types", OFFSET(thread_type), AV_OPT_TYPE_FLAGS,
         { .i64 = AVFILTER_THREAD_SLICE }, 0, INT_MAX, FLAGS, "thread_type" },
-        { "slice", NULL, 0, AV_OPT_TYPE_CONST, { .i64 = AVFILTER_THREAD_SLICE }, .unit = "thread_type" },
+        { "slice", NULL, 0, AV_OPT_TYPE_CONST, { .i64 = AVFILTER_THREAD_SLICE }, .flags = FLAGS, .unit = "thread_type" },
     { "enable", "set enable expression", OFFSET(enable_str), AV_OPT_TYPE_STRING, {.str=NULL}, .flags = FLAGS },
     { "threads", "Allowed number of threads", OFFSET(nb_threads), AV_OPT_TYPE_INT,
         { .i64 = 0 }, 0, INT_MAX, FLAGS },