fix doc/build_draft.sh run error

Add newer source directories to the destdir file tree so we
can include all sources referenced from opus_sources.mk.

Signed-off-by: Ralph Giles <giles@thaumas.net>
4 jobs for draft-willson in 4 minutes and 52 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #10315
docker
autoconf

00:04:52

passed #10316
docker
cmake

00:02:00

 
  External
passed continuous-integration/appveyor/branch

passed continuous-integration/appveyor/merge-request