Otherwise, tests dependencies are not considered for build.
Add an error in Makefile.inc so this won't happen again.

Cc: Cyrill Gorcunov <gorcunov@openvz.org>
Reported-by: Andrei Vagin <avagin@virtuozzo.com>
Signed-off-by: Dmitry Safonov <dima@arista.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>