nftables: disable dblatex check

On the systems with old dblatex (for example 3.0) the build step will
fail, because dblatex doesn't support '-q' parameter. This patch
overrides dblatex check.

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>