libaio: use alternative upstream URL

Since the kernel.org security problem and reinstallation, libaio is no
longer available from this site. Use snapshot.debian.net as an
alternative source.

[Peter: Add spaces around '=' while we're at it]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>