move /lib/modules/.../build as file into .deb

move /lib/modules/.../build as file into .deb

this symlink should not be created in the postinst,
but simply included in the pve-headers-xx .deb

this is a followup to the recently dropped dependency of
pve-headers-xx on pve-kernel-xx, and makes this more
consistent with how upstream (Ubuntu) handles the creation
of this symlink.