builddeb: really include objtool binary in headers package

On May 4th, Bjørn Mork provided patch 697bbc7b ("builddeb: include
objtool binary in headers package"). However, that one only works if
$srctree=$objtree, because the objtool binaries are not written to the
srctree, but to the objtree.

Signed-off-by: Wilfried Klaebe <linux-kernel@lebenslange-mailadresse.de>
Fixes: 697bbc7b ("builddeb: include objtool binary in headers package")
Signed-off-by: Michal Marek <mmarek@suse.com>