package/mpd: disable documentation

Disable documentation which is enabled by default since version 0.22 and
https://github.com/MusicPlayerDaemon/MPD/commit/2e73e605f78d2e6488e34465a8bfa9e4989a057f



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>