rtpenc_chain: Pass the initial time_base hint on to the chained muxer

In practice this hint is ignored - the rtp muxer always overwrites
the stream time base without taking the hint into account. But as
a general practice this is the correct way to pass a time base hint
on to a chained muxer.

This avoids warnings about using the codec time base as hint
being deprecated.

Signed-off-by: Martin Storsjö <martin@martin.st>