vkupload: unref vkimagebufferpool

Since the vkupload pool refacto, the pool
was not properly unrefed leading to a leak.

Fix for fd5c344fb6

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/5729>

subprojects/gst-plugins-bad/ext/vulkan/vkupload.c

@@ -1404,6 +1404,8 @@ gst_vulkan_upload_decide_allocation (GstBaseTransform * bt, GstQuery * query)
   else
     gst_query_add_allocation_pool (query, pool, size, min, max);
 
+  gst_object_unref (pool);
+
   return TRUE;
 }