add seccomp to DISTRO_FEATURES

Commit aff0243c added seccomp to the systemd PACKAGECONFIG. The
libseccomp recipe requires seccomp be a DISTRO_FEATURE.

Signed-off-by: Andrew Geissler <geissonator@yahoo.com>
Change-Id: Iee1d5e9b2efe8284454c0b5125d9de7b43c1bdb0