Makefiles: nuke $(SRC_DIR)

As all builds are done from top source dir, there is no need
to have SRC_DIR.

Reviewed-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>