package/docker-engine: libseccomp is mandatory

libseccomp is mandatory since bump to version 23.0.0 in commit
485b47e0:
https://github.com/moby/moby/pull/42501

Fixes:
 - https://bugs.buildroot.org/show_bug.cgi?id=15321



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>