d/rules: headers compile: run modules_prepare copy scripts & includes after

d/rules: headers compile: run modules_prepare copy scripts & includes after

The prepare/compile/install targets feel a bit mixed, so it's not
100% clear where this should happen.

But as the `.headers_compile_mark` already triggers various kernel
build targets with a correct kconfig setup, it is a good fit to add
the modules_prepare step (which is recommended to use when preparing
a out-of-three (OOT) module build environment like dkms expects)
there. As we can only copy (= install) the `scripts` directory
afterwards it follows that it needs to be moved afterwards. Moving
installing the `include` directory there is not really necessary but
it feels like a better place than the _prepare_ target and safes a
extra line, so move that over too.

In terms of actual changes to the built header package we get
additionally the, now generated, module.lds file.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>