BUILD: Fix build without samba
    
    The test test_ad_subdom should be compiled only if samba build is enabled.
    
    In file included from src/tests/cmocka/test_ad_subdomains.c:39:0:
    ./src/providers/ad/ad_subdomains.c:35:17: fatal error: ndr.h: No such file or directory
     #include <ndr.h>
                     ^
    compilation terminated.
    
    Reviewed-by: Sumit Bose <sbose@redhat.com>
    Reviewed-by: Fabiano Fidêncio <fidencio@redhat.com>