libmpd: change download URL

The old and new URLs are equivalent, but the new URL is not specific
to an older version of libmpd, which seems more logical.

[Thomas: update commit log.]

Signed-off-by: Jerzy Grzegorek <jerzy.grzegorek@trzebnica.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>