update wireguard to upstream version 0.0.20180809
Whilst the month old commit https://gitlab.labs.nic.cz/turris/openwrt/commit/265c4d4eb4c86f954501095e0e3d03e02ab27947 is not yet even merged to the test branch the upstream package https://github.com/lede-project/source/tree/master/package/network/services/wireguard | https://github.com/openwrt/openwrt/tree/master/package/network/services/wireguard has meantime been bumped to version 0.0.20180809