d/clean: Create file and add build generated files

This allows multiple builds without trashing the buildenv with modified
files, eg.: building with debuild.
Please note that gbp buildpackage is not affected by this because it does
build packages from another directory, ../build-area.

debian/clean

+ extensions/matches.man
+ extensions/targets.man
+ iptables/iptables-extensions.8
+ iptables/xtables-config-parser.h