build: have `make clean` remove dep files too

build: have `make clean` remove dep files too

While changing branches, one can hit errors like:

make[2]: ***   CC       libipt_CLUSTERIP.oo
No hay ninguna regla para construir el objetivo
`../include/net/netfilter/nf_nat.h', necesario para
`libipt_DNAT.oo'. Alto.

Pablo thinks dep files should be removed on `make clean`, and I
concur. (JFI, Note that native automake would not clear its ".deps"
directory.) Keep the "distclean: clean" line to keep invocations by
automake from the parent directory working.

Reported-by: Pablo Neira Ayuso <pablo@netfilter.org>