MAINTAINERS: add David Ahern to IPv4/IPv6 maintainers

David has been the de-facto maintainer for much of the IP code
for the last couple of years, let's make it official.

Link: https://lore.kernel.org/r/20210122173220.3579491-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

MAINTAINERS

@@ -12413,6 +12413,7 @@ F:	tools/testing/selftests/net/ipsec.c
 NETWORKING [IPv4/IPv6]
 M:	"David S. Miller" <davem@davemloft.net>
 M:	Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>
+M:	David Ahern <dsahern@kernel.org>
 L:	netdev@vger.kernel.org
 S:	Maintained
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net.git