samples: nrf: onoff_level_lighting_vnd_app: fix build with mcumgr

There is an optional mcumgr enabled prj_smp_svr.conf configuration
file. Using it results in following errors:

  .../src/smp_svr.c:12:10: fatal error: mgmt/buf.h: No such file or \
  directory
     12 | #include <mgmt/buf.h>
        |

Fix include paths to keep sample buildable with mcumgr enabled.

Fixes: c200b1c5 ("mgmt: Move mcumgr into its own folder")
Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>