TX counters not updated on wireguard interface

[Jason A. Donenfeld]

Hey again,

I've committed a fix [1] for this which will be out in the next
snapshot. I've also added a test [2] for this to the automated test
suite on https://build.wireguard.io/ so that if the bug ever
resurfaces, it will be caught. Before reading [1], be sure to wear
protective eyewear, as your pupils are likely to start bleeding at the
sight of horrendous compatibility ifdef hackery.

Jason

[1] https://git.zx2c4.com/WireGuard/commit/?id=4c294b7ec676105abc3abb3a3dd65a63a3466eb2
[2] https://git.zx2c4.com/WireGuard/commit/?id=5437b87afcfed371b31eff25f15663f3d43ca065