wireguard: bump to 0.0.20190601

wireguard: bump to 0.0.20190601

There was an issue with the backport compat layer in yesterday's snapshot,
causing issues on certain (mostly Atom) Intel chips on kernels older than
4.2, due to the use of xgetbv without checking cpu flags for xsave support.
This manifested itself simply at module load time. Indeed it's somewhat tricky
to support 33 different kernel versions (3.10+), plus weird distro
frankenkernels.

If OpenWRT doesn't support < 4.2, you probably don't need to apply this.
But it also can't hurt, and probably best to stay updated.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
(cherry picked from commit 593b487538079f2a22300f3f22ffb21b20da36a0)