openbmc: enable libseccomp for systemd

This change required as a part of privilege separation work:
  https://github.com/openbmc/openbmc/issues/3383

Seccomp support enables sandboxing in systemd:
  https://www.freedesktop.org/software/systemd/man/systemd.exec.html#Sandboxing



Signed-off-by: Anton D. Kachalov <gmouse@google.com>
Change-Id: If7ff257103c4aa40dae5f64256bf60d8a30fbf59