package/redis: Add systemd service

redis.service originates from the redis package in Arch Linux, at
https://projects.archlinux.org/svntogit/community.git/tree/trunk/redis.service?h=packages/redis&id=b4907d2344855caeadf6c69bc01c403a95f8a4f3

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Signed-off-by: Martin Bark <martin@barkynet.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>