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: NNicolas de Pesloüan <nicolas.2p.debian@free.fr>
Tested-by: NNikolai Kondrashov <spbnick@gmail.com>
Acked-by: Nmaximilian attems <max@stro.at>
Signed-off-by: NMichal Marek <mmarek@suse.cz>