mpd: fix fetch failure for 0.18.x

MPD stable download location will contain only the latest
stable branch, so fetch will not work in case of an older
OE recipe. i.e., mpd 0.18.x recipe will no longer work
after 0.19 branch is released (as it is the case now).

Signed-off-by: Cristian Iorga <ubik3000@gmail.com>
Signed-off-by: Cristian Iorga <cristian.iorga@intel.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>