debian: fix distclean issue

The build system of nftables doesn't implement the distclean op.

In previous commit, I solved this by overriding dh_auto_clean, but this
was invalid.

A better approach is to patch the Makefile.rules.in file and add a proper
distclean operation.

Signed-off-by: Arturo Borrero Gonzalez <arturo.borrero.glez@gmail.com>