dkms module build fails since at least 20191226 on CentOS

[Silvan Nagl]

Hi again,

since the last WireGuard upgrade (wireguard-0.0.20191226) the dkms
module does not build anymore. I just noticed that my CentOS box still
runs on 0.0.20191206 so it appeared to work at firs glance but actually
it just uses the old dkms module which was not overwritten by the new
installation because it failed to build.

Here is make all output:

[root at static wireguard-0.0.20191226]# make all
  CC [M]  /usr/src/wireguard-0.0.20191226/main.o
  CC [M]  /usr/src/wireguard-0.0.20191226/noise.o
  CC [M]  /usr/src/wireguard-0.0.20191226/device.o
  CC [M]  /usr/src/wireguard-0.0.20191226/peer.o
  CC [M]  /usr/src/wireguard-0.0.20191226/timers.o
  CC [M]  /usr/src/wireguard-0.0.20191226/queueing.o
  CC [M]  /usr/src/wireguard-0.0.20191226/send.o
  CC [M]  /usr/src/wireguard-0.0.20191226/receive.o
In file included from ./include/linux/init.h:5,
                 from /usr/src/wireguard-0.0.20191226/compat/compat.h:50,
                 from <command-line>:
/usr/src/wireguard-0.0.20191226/receive.c: In function ‘wg_packet_receive’:
/usr/src/wireguard-0.0.20191226/receive.c:578:17: error: implicit
declaration of function ‘rng_is_initialized’; did you mean
‘zone_is_initialized’? [-Werror=implicit-function-declaration]
       unlikely(!rng_is_initialized())) {
                 ^~~~~~~~~~~~~~~~~~
./include/linux/compiler.h:77:42: note: in definition of macro ‘unlikely’
 # define unlikely(x) __builtin_expect(!!(x), 0)
                                          ^
cc1: some warnings being treated as errors
make[2]: *** [scripts/Makefile.build:316:
/usr/src/wireguard-0.0.20191226/receive.o] Error 1
make[1]: *** [Makefile:1528: _module_/usr/src/wireguard-0.0.20191226]
Error 2
make: *** [Makefile:38: module] Error 2

Greetings,

Silvan