kbuild: builddeb: always make modules_install, to install modules.builtin*

Even for a non-modular kernel, the kernel builds modules.builtin and
modules.builtin.modinfo, with information about the built-in modules.
Tools such as initramfs-tools need these files to build a working
initramfs on some systems, such as those requiring firmware.

Now that `make modules_install` works even in non-modular kernels and
installs these files, unconditionally invoke it when building a Debian
package.

Signed-off-by: Josh Triplett <josh@joshtriplett.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>