package/mpd: fix static build issue with alsa

mpd forgets to link the archive libmixer_plugins.a against libasound leading
to undefined references in a static context.

Add a patch sent upstream to fix:
http://autobuild.buildroot.net/results/f10/f10c6ea2d29bbcef8f33fc9ba1430e99edca895d/
http://autobuild.buildroot.net/results/312/31212c484f452fe4ea6ae084d3edefc4580830d8/



Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>