openvswitch: Export lockdep_ovsl_is_held to modules.

ERROR: "lockdep_ovsl_is_held" [net/openvswitch/vport-gre.ko] undefined!

Reported-by: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>