OpenVswitch GCC 4.9 build fix

Fixes this issue: https://github.com/openwrt/packages/issues/457

The patch has also been upstreamed here: https://github.com/openvswitch/ovs/pull/18
But, that's the latest trunk of OVS, and we use version 2.3 which is the last stable.
So, it will be a while until we can pull this patch out.

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>