net/rds/Kconfig: RDS should depend on IPV6

Build error, implicit declaration of function __inet6_ehashfn shows up
When RDS is enabled but not IPV6.
net/rds/connection.c: In function ‘rds_conn_bucket’:
net/rds/connection.c:67:9: error: implicit declaration of function ‘__inet6_ehashfn’; did you mean ‘__inet_ehashfn’? [-Werror=implicit-function-declaration]
  hash = __inet6_ehashfn(lhash, 0, fhash, 0, rds_hash_secret);
         ^~~~~~~~~~~~~~~
         __inet_ehashfn

Current code adds IPV6 as a depends on in config RDS.

Fixes: eee2fa6a ("rds: Changing IP address internal representation to struct in6_addr")
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Acked-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>