Fix T101775: grease pencil keyframe not filtered in dopesheet summary

Grease pencil data keyframes were listed twice in the summary.

First by the generic object data listing,
which did not handle properly grease pencil objects,
and did not account for the grease pencil filter.
Second by the specific grease pencil function.

Now only the second call is made,
and the filter hides keyframes in summary as well.

Reviewed By : Jeroen Bakker, Falk David

Differential Revision: https://developer.blender.org/D16369

source/blender/editors/animation/anim_filter.c

@@ -2826,7 +2826,7 @@ static size_t animdata_filter_dopesheet_ob(
     }
 
     /* object data */
-    if (ob->data) {
+    if ((ob->data) && (ob->type != OB_GPENCIL)) {
       tmp_items += animdata_filter_ds_obdata(ac, &tmp_data, ads, ob, filter_mode);
     }