Add vorbis dependencies when building examples.

The decoder_example program calls floor() which requires
its own -lm on the link line to resolve the local reference.

It seems on most platforms this was shadowed by the transitive
dependency in the vorbis libraries, but it results in unddefine
reference error on e.g. debian mipsel.
2 jobs for example-libm in 1 minute and 52 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #10558
docker
autoconf

00:01:52

passed #10559
docker
cmake

00:00:38