rtpfunnel: fix extmap handling on accept-caps

rtpfunnel: fix extmap handling on accept-caps

Follow-up on 97d83056b315c56834eaa6776ae4c6a0848b5ef9, only check
for intersection with the current srccaps when checking if a sinkpad
can accept caps.

I must have been lucky in my firefox testing then, and always entered
the code path with audio getting negotiated first, thus not failing
the is_subset check when srccaps had been negotiated as
application/x-rtp, and an accept-caps query was made for the video
caps with a defined extmap.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1384>