ppc64le: remove seccomp from Dockerfile

Removes the seccomp buildtag when building runc.
Because seccomp isn't currently being built, this would cause
the build to fail.

Signed-off-by: Christopher Jones <tophj@linux.vnet.ibm.com>
(cherry picked from commit 2caf09d37b5865882dfa8c60d5362fd6d2ba54fd)