ARM: multi_v7_defconfig: Enable SECCOMP

Systemd already supports seccomp.  It seems some distros are building
systemd with seccomp, e.g. Arch on ARM, thus leading to boot errors:

systemd-logind.service: Failed at step SECCOMP spawning /usr/lib/systemd/systemd-logind: Invalid argument

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Reviewed-by: Javier Martinez Canillas <javier@osg.samsung.com>