mac_ddb: Only include the vnet validator in VIMAGE kernels.

This fixes the build of at least i386 MINIMAL which was failing with
the error:

sys/security/mac_ddb/mac_ddb.c:200:15: error: use of undeclared identifier 'vnet'; did you mean 'int'?

if ((void *)vnet == (void *)addr)
            ^~~~
            int

Sponsored by: DARPA