builddeb: Pass the kernel:debarch substvar to dpkg-genchanges

At the end of "make bindeb-pkg" I noticed the following warning:

dpkg-genchanges: warning: unknown substitution variable ${kernel:debarch}

It turns out that since dpkg version 1.19.0 dpkg-genchanges honors
substitution variables in the Description field, while earlier
versions silently left them alone, see https://bugs.debian.org/856547

.

The result is an incomplete description of the linux-headers package
in the generated .changes file.  Fix it by passing the kernel:debarch
substitution variable to dpkg-genchanges.

Signed-off-by: Sven Joachim <svenjoac@gmx.de>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>