builddeb: allow selection of .deb compressor

Select deb compression using KDEB_COMPRESS make variable. This allows to
use gzip compression for local or test builds, and that's way faster
than now-default xz compression.

Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>