builddeb: make headers package thinner

Remove a bunch of files not used during external module builds:
 - foreign architecture headers
 - subtree Makefiles
 - Kconfig files
 - perl scripts

On amd64 system this looses a third of the resulting .deb size.

Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>