configure: fix seccomp check

Currently, if libseccomp is missing but the user explicitly requested
seccomp support using --enable-seccomp, configure silently ignores the
situation and disables seccomp support.

This is unlike all other tests that explicitly fail in such situation.

Fix that.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>
