mpd: fix vorbis build failure

Forcibly disable the vorbis encoder support when the vorbis option isn't
enabled to avoid autodetection.
Said autodetection has a build bug that it won't properly set libvorbis
build options when the decoder is disabled. Fixes:
http://autobuild.buildroot.net/results/a46/a46b4c97fe126b23a96051a8bf1243a7f4935434/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>