libvorbis: fix LIBVORBIS_SITE

The LIBVORBIS_SITE included a reference to $(LIBVORBIS-SOURCE), which
doesn't exist. And anyway was useless because the <pkg>_SITE variable
should not contain the name of the tarball.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>