samples: nrf: onoff_level_lighting_vnd_app: build with mcumgr

There is an additional prj_smp_svr.conf configuration file provided,
which builds an image with mcuboot support. Enable it in sample.yaml,
so sanitycheck can detect build failures in future.

Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>