mpd.service: depend on network.target

Since some configurations use the "bind_to_address" option in their
/etc/mpd.conf, the systemd service file must wait for the
network.target or else mpd will start before it and thus fail due to
no iface.

mpd.service.in

@@ -1,6 +1,6 @@
 [Unit]
 Description=Music Player Daemon
-After=sound.target
+After=network.target sound.target
 
 [Service]
 ExecStart=@prefix@/bin/mpd --no-daemon