kbuild, deb-pkg: improve Source field

The Source: field is defined as the source package in the package
archive from which a binary packages are built. As deb-pkg does not
generate a source package, we should avoid to use any existing source
packages here.

Signed-off-by: Frans Pop <elendil@planet.nl>
Acked-by: maximilian attems <max@stro.at>
Cc: Andres Salomon <dilinger@debian.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>