builddeb: Fix rootless build in setuid/setgid directory

Building 5.10-rc1 in a setgid directory failed with the following
error:

dpkg-deb: error: control directory has bad permissions 2755 (must be
>=0755 and <=0775)

When building with fakeroot, the earlier chown call would have removed
the setgid bits, but in a rootless build they remain.

Fixes: 3e854180 ("builddeb: Enable rootless builds")
Cc: Guillem Jover <guillem@hadrons.org>
Signed-off-by: Sven Joachim <svenjoac@gmx.de>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>