Drop the libseccomp required version back to 2.1.0, restoring the ability
to build w/ --enable-seccomp on Ubuntu 14.04.

Commit 4cc47f8b tightened the dependency
on libseccomp from version 2.1.0 to 2.1.1. This broke building on Ubuntu
14.04, the current Ubuntu LTS release. The commit message didn't mention
any specific functional need for 2.1.1, just that it was the most recent
stable version at the time. I reviewed the changes between 2.1.0 and 2.1.1,
but it looks like that update just contained minor fixes and cleanups - no
obvious (to me) new interfaces or critical bug fixes.

Signed-off-by: dann frazier <dann.frazier@canonical.com>
Acked-by: Eduardo Otubo <eduardo.otubo@profitbricks.com>