Build failing for armada38x / DSM 7.0

[paullbn]

Description
I cant build the package for my DS216J (armada38x) running DSM 7.0

Steps to reproduce

sudo docker build -t synobuild .
sudo docker run --rm --privileged --env PACKAGE_ARCH=armada38x --env DSM_VER=7.0 -v $(pwd)/artifacts:/result_spk synobuild

...

/source/WireGuard/wireguard-linux-compat-1.0.20211208/src/compat/siphash/siphash.c: In function '__siphash_aligned':
/source/WireGuard/wireguard-linux-compat-1.0.20211208/src/compat/siphash/siphash.c:80:36: error: 'fallthrough' undeclared (first use in this function)
  case 7: b |= ((u64)end[6]) << 48; fallthrough;
                                    ^~~~~~~~~~~
/source/WireGuard/wireguard-linux-compat-1.0.20211208/src/compat/siphash/siphash.c:80:36: note: each undeclared identifier is reported only once for each function it appears in
/source/WireGuard/wireguard-linux-compat-1.0.20211208/src/compat/siphash/siphash.c: In function '__siphash_unaligned':
/source/WireGuard/wireguard-linux-compat-1.0.20211208/src/compat/siphash/siphash.c:112:36: error: 'fallthrough' undeclared (first use in this function)
  case 7: b |= ((u64)end[6]) << 48; fallthrough;
                                    ^~~~~~~~~~~
/source/WireGuard/wireguard-linux-compat-1.0.20211208/src/compat/siphash/siphash.c: In function '__hsiphash_aligned':
/source/WireGuard/wireguard-linux-compat-1.0.20211208/src/compat/siphash/siphash.c:429:36: error: 'fallthrough' undeclared (first use in this function)
  case 3: b |= ((u32)end[2]) << 16; fallthrough;
                                    ^~~~~~~~~~~
/source/WireGuard/wireguard-linux-compat-1.0.20211208/src/compat/siphash/siphash.c: In function '__hsiphash_unaligned':
/source/WireGuard/wireguard-linux-compat-1.0.20211208/src/compat/siphash/siphash.c:451:36: error: 'fallthrough' undeclared (first use in this function)
  case 3: b |= ((u32)end[2]) << 16; fallthrough;
                                    ^~~~~~~~~~~
scripts/Makefile.build:312: recipe for target '/source/WireGuard/wireguard-linux-compat-1.0.20211208/src/compat/siphash/siphash.o' failed
make[3]: *** [/source/WireGuard/wireguard-linux-compat-1.0.20211208/src/compat/siphash/siphash.o] Error 1
Makefile:1243: recipe for target '_module_/source/WireGuard/wireguard-linux-compat-1.0.20211208/src' failed
make[2]: *** [_module_/source/WireGuard/wireguard-linux-compat-1.0.20211208/src] Error 2
Makefile:26: recipe for target 'module' failed
make[1]: *** [module] Error 2
make[1]: Leaving directory '/source/WireGuard/wireguard-linux-compat-1.0.20211208/src'
Makefile:72: recipe for target 'wireguard-linux-compat-1.0.20211208/src/wireguard.ko' failed
make: *** [wireguard-linux-compat-1.0.20211208/src/wireguard.ko] Error 2
[Error] Build project fail!
Time cost: 00:00:27 [Build-->WireGuard]
[INFO] Build WireGuard finished!

----------------- Time cost statistics -----------------
Time cost: 00:00:27 [Build-->WireGuard]

########################################################
      Error(s) occurred on project "WireGuard"
########################################################
1 projects, 1 failed, 0 blocked.

[Error] Check [/logs/error.build] for fixing errors.
Install log
===========
cat: /build_env/ds.armada38x-7.0/logs.install: No such file or directory

Expected behavior
Build success

Synology NAS model
DS216j

Do you have an idea of what could have gone wrong?
Paul