netfilter: ebtables: fix ebtables build dependency

The configuration of ebtables shouldn't depend on
CONFIG_BRIDGE_NETFILTER, only on CONFIG_NETFILTER.

Reported-by: Sbastien Laveze <slaveze@gmail.com>
Signed-off-by: Bart De Schuymer <bdschuym@pandora.be>
Signed-off-by: Patrick McHardy <kaber@trash.net>