kbuild: deb-pkg: fix warning when CONFIG_DEBUG_INFO is unset

Creating a Debian package without CONFIG_DEBUG_INFO produces
a warning that no debug package was created.

This patch excludes the debug package from the control file,
if no debug package is created by this configuration.

Signed-off-by: Reinhard Karcher <reinhard.karcher@gmx.net>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>