d/rules: kconfig: enable SQUASHFS_DECOMP_MULTI_PERCPU

d/rules: kconfig: enable SQUASHFS_DECOMP_MULTI_PERCPU

Debian did so since 5.10~rc7-1~exp1 and ubuntu only disabled it due
some concerns about "high" memory usage on many-core systems[0], high
is to be seen relative here as its 26 MiB on 208 cores[1] and only
matters for ubuntu as due to their snaps they may have a lot of
active squashfs mounts.

Proxmox projects do not use snaps, or other things that uses squashfs
instances a tall besides the installer. While some users may use a
few it is unlikely to cause much problems (a few 100 MiB should not
be a big problem on a server with hundreds of online cores.

Any how, to speed up decompression in our installer and use a similar
setting as Debian, the distro we're most similar too, enable this
Kconfig knob.

[0]: https://bugs.launchpad.net/snappy/+bug/1636847
[1]: https://bugs.launchpad.net/snappy/+bug/1636847/comments/21

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>