package/mpd: opus plugin needs libogg as well

Fixes http://autobuild.buildroot.net/results/492/4924194292a428a295ded2f7aa0b9a3f81ed1b4a/



The opus decoder plugin depends on libopus and libogg.

[Peter: reworded commit message]
Signed-off-by: Jörg Krause <jkrause@posteo.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>