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>

Author: Florian Westphal

MAINTAINERS

@@ -17480,6 +17480,7 @@ NETFILTER
 M:	Pablo Neira Ayuso <pablo@netfilter.org>
 M:	Jozsef Kadlecsik <kadlec@netfilter.org>
 M:	Florian Westphal <fw@strlen.de>
+R:	Phil Sutter <phil@nwl.cc>
 L:	netfilter-devel@vger.kernel.org
 L:	coreteam@netfilter.org
 S:	Maintained