kbuild: Generate modules.builtin in make modules_install

The previous approach didn't work if one did
make modules && make modules_install
Add modules.builtin as dependency of _modinst_, which is the target that
actually needs the file.

Reported-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>