package/mpd: add option to enable or disable OSS support

By default, the OSS support is enabled, which may not necessarily be
useful on most Linux systems that use ALSA.

[Thomas: tweak commit title and log.]

Signed-off-by: Jörg Krause <jkrause@posteo.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>