mpd: Add missing dependency on audiofile

Otherwise we get errors like

| Collected errors:
|  * satisfy_dependencies_for: Cannot satisfy the following dependencies for mpd:
|  *    libaudiofile0 (>= 0.2.7) *
|  * opkg_install_cmd: Cannot install package mpd.

Signed-off-by: Khem Raj <raj.khem@gmail.com>