builddeb: fix file permissions before packaging

Builddep is not very explicit about file permissions. Actually the file
permissions in the package are largely influenced by the umask of the
user cloning the git and building the package. If that umask does not
set go+r the resulting linux-headers package will prevent non-root users
from building out-of-tree modules. And that is probably just one
unexpected effect.
Being a packaging/install tool builddep should make sure the file
permissions are set correctly and not just derived from a value that is
never checked.

This patch sets ugo read permissions for all packaged files and derives
the executable bit for directories and executables from the file-owner.

Signed-off-by: Henning Schild <henning.schild@siemens.com>
Signed-off-by: Michal Marek <mmarek@suse.com>