deb-pkg: Fix building outside of source tree (O=...).

When building linux-headers package using deb-pkg, builddeb erroneously assume
current directory is the source tree. This is not true if building in another
directory, using make O=... deb-pkg.

This patch fix this problem.

Signed-off-by: Nicolas de Pesloüan <nicolas.2p.debian@free.fr>
Tested-by: Nikolai Kondrashov <spbnick@gmail.com>
Acked-by: maximilian attems <max@stro.at>
Signed-off-by: Michal Marek <mmarek@suse.cz>