kernel: Add kmod-sch-cake-virtual intermediate package

kernel: Add kmod-sch-cake-virtual intermediate package

As reported in https://github.com/openwrt/packages/issues/12072, the
imagebuilder fails due to a dependency resolution error when the userspace
packages are built using a target that has a different kernel version than
that which is being run. To resolve this, add a virtual kernel package with
the conditional dependency currently used in sqm-scripts. The idea is to
move the sqm-scripts dependency to this virtual package, which hopefully
should be consistent with the actual kernel module being built.

Signed-off-by: Toke Høiland-Jørgensen <toke@toke.dk>