net: ethernet: rocker: Add dependency to CONFIG_BRIDGE in Kconfig

In a configuration with CONFIG_BRIDGE set to 'm' and CONFIG_ROCKER
set to 'y', undefined references occur at link time:

> drivers/built-in.o: In function `rocker_port_fdb_learn_work':
> /home/jim/linux/drivers/net/ethernet/rocker/rocker.c:3014: undefined
> reference to `br_fdb_external_learn_del'
> /home/jim/linux/drivers/net/ethernet/rocker/rocker.c:3016: undefined
> reference to `br_fdb_external_learn_add'

This patch fixes these by declaring CONFIG_ROCKER as being dependent
on CONFIG_BRIDGE.

Reported-by: Jim Davis <jim.epost@gmail.com>
Signed-off-by: Andreas Ruprecht <rupran@einserver.de>
Acked-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: David S. Miller <davem@davemloft.net>