ovs: Include optimizations for json string serialization.

Updating OVS submodule to include following patch:
  748010ff304b ("json: Optimize string serialization.")

It provides faster json string serialization function that
is used for every jsonrpc message, hence slightly improves
performance of all communications.

Signed-off-by: Ilya Maximets <i.maximets@ovn.org>
Acked-by: Dumitru Ceara <dceara@redhat.com>
Signed-off-by: Numan Siddique <numans@ovn.org>