MAINTAINERS: add Phil as netfilter reviewer

Phil has contributed to netfilter with features, fixes and patch reviews
for a long time.  Make this more formal and add Reviewer tag.

Acked-by: Jozsef Kadlecsik <kadlec@netfilter.org>
Acked-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Florian Westphal <fw@strlen.de>