wireguard: bump to 1.0.20200623

wireguard: bump to 1.0.20200623

* compat: drop centos 8.1 support as 8.2 is now out

Of note, as well, is that we now have both RHEL7 and RHEL8 in our CI at
<https://www.wireguard.com/build-status/>.

* Kbuild: remove -fvisibility=hidden from cflags

This fixes an issue when compiling wireguard as a module for ARM kernels in
THUMB2 mode without the JUMP11 workaround.

* noise: do not assign initiation time in if condition

Style fix.

* device: avoid circular netns references

Fixes a circular reference issue with network namespaces.

* netns: workaround bad 5.2.y backport

This works around a back backport in the 5.2.y series.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>