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 an
unddefined reference error on e.g. debian 10 mipsel.

Signed-off-by: Mark Harris <mark.hsj@gmail.com>
2 jobs for example-libm in 1 minute and 55 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #10560
docker
autoconf

00:01:55

passed #10561
docker
cmake

00:00:36