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>

MAINTAINERS

@@ -9936,6 +9936,7 @@ F:	net/rfkill/
 
 RHASHTABLE
 M:	Thomas Graf <tgraf@suug.ch>
+M:	Herbert Xu <herbert@gondor.apana.org.au>
 L:	netdev@vger.kernel.org
 S:	Maintained
 F:	lib/rhashtable.c