draco: Add dummy sources to draco, dracodec, and dracoenc.
draco: Add dummy sources to draco, dracodec, and dracoenc.

This is a workaround for two issues with CMake's Xcode generator:
- Setting the LINK_LANGUAGE property does not work.
- Library targets that only combine other object files do not build in
  Xcode because of some other issue in the generator. They don't fail
  at build time: Xcode just does not build them.

The combined behavior of the two issues forces addition of dummy source
files to the listed targets.

Fixes issue https://github.com/google/draco/issues/593

google/draco,louquillio/draco,louquillio/draco,google/draco,google/draco,louquillio/draco,louquillio/draco,google/draco,google/draco,louquillio/draco,google/draco,google/draco,louquillio/draco