kbuild: deb-pkg: Do not install maint scripts for arch 'um'

Stop installing Debian maintainer scripts when building a
user-mode-linux Debian package.

Debian maintainer scripts are used for e.g. requesting rebuilds of
initrd, rebuilding DKMS modules and updating of grub configuration.  As
all of this is not relevant for UML but also may lead to failures while
processing the kernel hooks, do no more install maintainer scripts for
the UML package.

Suggested-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Nicolas Schier <nicolas@fjasle.eu>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>