[21.02] umdns: does not start because of missing seccomp features

[Nick]

To reproduce:
1) Build 21.02 image with imagebuiler
2) opkg update && opkg install umnds
3) umdns will not start

The seccomp file is existing ( /etc/seccomp/umdns.json ), and so the line:
https://github.com/openwrt/openwrt/blob/openwrt-21.02/package/network/services/umdns/files/umdns.init#L36

However, the packages are missing:
- procd-ujail
- procd-seccomp

Installing procd-seccomp is fixing the issue. I would do a PR, but I'm 
unsure what is the correct way of solving this issue.

Bests,
Nick