ovs-router: Define stub for ovs_router_unixctl_register()

ovs_router_unixctl_register() is called from ofproto layer.
But is not defined for all platform. Following patch define
stub to fix compilation error.

Reported-by: Eitan Eliahu <eliahue@vmware.com>
Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Acked-by: Jarno Rajahalme <jrajahalme@nicira.com>-