kbuild, deb-pkg: fix Provides field

kernel-image naming has been dropped for the Lenny release
and was only transitional for Etch.

As it builds modules it provides linux-modules-$version.

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