libs: context: use correct printing modifier
libs: context: use correct printing modifier

GstVaapiID is an alias of gsize, thus its modifier is platform
dependant.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer-vaapi/-/merge_requests/340>

GStreamer/gstreamer-vaapi,ceyusa/gstreamer-vaapi,GStreamer/gstreamer-vaapi,ceyusa/gstreamer-vaapi