alsa: add http mirror in the backported alsa recipes

alsa: add http mirror in the backported alsa recipes

Unfortunately ftp doesn't work for me (looks like a firewall issue)
and the default poky mirror at http://downloads.yoctoproject.org/mirror/sources/
doesn't contain alsa 1.1.6, so the build fails.

I think this will be useful to other people with firewall issues.

Change-Id: Ic6744f38ac8c007ed99b69fb410526b6c4ca8733
Signed-off-by: George Kiagiadakis <george.kiagiadakis@collabora.com>
(cherry picked from commit 370c5183bf1cc13ca8e0d2b50c7656175652de3e)