libavformat: Fix standalone compilation of the webm muxer

The missing object file was added to the matroska muxer already, but
not to the webm muxer.

Signed-off-by: Martin Storsjö <martin@martin.st>
rtpdec: Fix the alphabetical ordering in registering depacketizers

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