wireguard: bump to 20191226

wireguard: bump to 20191226

As announced on the mailing list, WireGuard will be in Linux 5.6. As a
result, the wg(8) tool, used by OpenWRT in the same manner as ip(8), is
moving to its own wireguard-tools repo. Meanwhile, the out-of-tree
kernel module for kernels 3.10 - 5.5 moved to its own wireguard-linux-
compat repo. Yesterday, releases were cut out of these repos, so this
commit bumps packages to match. Since wg(8) and the compat kernel module
are versioned and released separately, we create a wireguard-tools
Makefile to contain the source for the new tools repo. Later, when
OpenWRT moves permanently to Linux 5.6, we'll drop the original module
package, leaving only the tools. So this commit shuffles the build
definition around a bit but is basically the same idea as before.

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