kbuild: deb-pkg: move 'make headers' to build-arch

Strictly speaking, 'make headers' should be a part of build-arch
instead of binary-arch.

'make headers' constructs ready-to-copy UAPI headers in the kernel
directory.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <n.schier@avm.de>