MAINTAINERS: Add extra rhashtable maintainer

As I'm working actively on rhashtable it helps if people CCed me
when they work on in.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Acked-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>