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>