builddeb: Enable rootless builds

This makes it possible to build the Debian packages without requiring
(pseudo-)root privileges, when the build drivers support this mode
of operation.

See-Also: /usr/share/doc/dpkg/rootless-builds.txt.gz
Signed-off-by: Guillem Jover <guillem@hadrons.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>