d/rules: headers: remove any .cmd files from the kbuild system

d/rules: headers: remove any .cmd files from the kbuild system

Those files are generated by the `if_changed` macro from
scripts/Kbuild.include and are not really useful or interesting for
being shipped in the header packages and other distros (checked
Debian and Ubuntu) do not seem to ship those at all..

So, lets prune them to reduce shipped files dramatically, without
losing, well, anything.

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