Add cgroups v2 dependency from backports
From LAVA release 2022.01 onward requirements of package "lava-dispatcher-host" cannot be resolved on Debian Bullseye without backports.
Affected requirement is "python3-bpfcc" (>= 0.21) which is available in Bullseye as 0.18.0+ds-2 (and as 0.22.0+ds-2~bpo11+1 in backports) as of writing this patch.
Patch that introduced this requirement: 180cb3c1
APT repository on lavasoftware.org supplies updated "python3-bpfcc"
(0.21.0+ds-7bpo11+1lava1).
Signed-off-by: Paweł Wieczorek pawiecz@collabora.com