mpd: forcibly disable oggflac without flac

If flac and libogg were both built but flac isn't selected in mpd it
will be picked up anyway.
Forcibly disable oggflac support if flac support is disabled for
consistency.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>