kbuild: rpm-pkg: run modules_install for non-modular kernel

For the same reason as commit 4243afdb ("kbuild: builddeb: always
make modules_install, to install modules.builtin*"), run modules_install
even when CONFIG_MODULES=n to install modules.builtin*.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>