kbuild: Fix link to headers in 'make deb-pkg'

The Link to the kernel header files in the debian packages
point to the original build directory. This is a bad choice
if the packages were installed on a different machine. Fix
this in by manually re-creating the link in the builddeb
script.

Cc: maximilian attems <max@stro.at>
Cc: debian-kernel@lists.debian.org
Cc: linux-kbuild@vger.kernel.org
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
Reviewed-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Michal Marek <mmarek@suse.cz>