rtpdec: Send a valid "delay since SR" value in the RTCP RR packets

Previously, we always signalled a zero time since the last RTCP
SR, which is dubious.

The code also suggested that this would be the difference in
RTP NTP time units (32.32 fixed point), while it actually is
in in 1/65536 second units. (RFC 3550 section 6.4.1)

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