Build failures of 20250718 with Debian 11 (Linux 5.10)

``` + cat /build/dest-debian11/make.log DKMS make.log for ovpn-dco-0.2.20250718-2+bullseye for kernel 5.10.0-35-amd64 (x86_64) Tue Jul 22 13:32:58 UTC 2025 /var/lib/dkms/ovpn-dco/0.2.20250718-2+bullseye/build/gen-compat-autoconf.sh /var/lib/dkms/ovpn-dco/0.2.20250718-2+bullseye/build/compat-autoconf.h make -C /lib/modules/5.10.0-35-amd64/build M=/var/lib/dkms/ovpn-dco/0.2.20250718-2+bullseye/build PWD=/var/lib/dkms/ovpn-dco/0.2.20250718-2+bullseye/build REVISION=0.2.20250718-2+bullseye CONFIG_OVPN_DCO_V2=m INSTALL_MOD_DIR=updates/ modules make[1]: Entering directory '/usr/src/linux-headers-5.10.0-35-amd64' CC [M] /var/lib/dkms/ovpn-dco/0.2.20250718-2+bullseye/build/drivers/net/ovpn-dco/main.o CC [M] /var/lib/dkms/ovpn-dco/0.2.20250718-2+bullseye/build/drivers/net/ovpn-dco/bind.o CC [M] /var/lib/dkms/ovpn-dco/0.2.20250718-2+bullseye/build/drivers/net/ovpn-dco/crypto.o CC [M] /var/lib/dkms/ovpn-dco/0.2.20250718-2+bullseye/build/drivers/net/ovpn-dco/ovpn.o CC [M] /var/lib/dkms/ovpn-dco/0.2.20250718-2+bullseye/build/drivers/net/ovpn-dco/peer.o CC [M] /var/lib/dkms/ovpn-dco/0.2.20250718-2+bullseye/build/drivers/net/ovpn-dco/sock.o CC [M] /var/lib/dkms/ovpn-dco/0.2.20250718-2+bullseye/build/drivers/net/ovpn-dco/stats.o CC [M] /var/lib/dkms/ovpn-dco/0.2.20250718-2+bullseye/build/drivers/net/ovpn-dco/netlink.o /var/lib/dkms/ovpn-dco/0.2.20250718-2+bullseye/build/drivers/net/ovpn-dco/peer.c: In function 'ovpn_peer_keepalive_set': /var/lib/dkms/ovpn-dco/0.2.20250718-2+bullseye/build/drivers/net/ovpn-dco/peer.c:306:3: error: implicit declaration of function 'timer_delete'; did you mean 'timer_reduce'? [-Werror=implicit-function-declaration] 306 | timer_delete(&peer->keepalive_xmit); | ^~~~~~~~~~~~ | timer_reduce CC [M] /var/lib/dkms/ovpn-dco/0.2.20250718-2+bullseye/build/drivers/net/ovpn-dco/crypto_aead.o CC [M] /var/lib/dkms/ovpn-dco/0.2.20250718-2+bullseye/build/drivers/net/ovpn-dco/pktid.o cc1: some warnings being treated as errors make[4]: *** [/usr/src/linux-headers-5.10.0-35-common/scripts/Makefile.build:291: /var/lib/dkms/ovpn-dco/0.2.20250718-2+bullseye/build/drivers/net/ovpn-dco/peer.o] Error 1 make[4]: *** Waiting for unfinished jobs.... make[3]: *** [/usr/src/linux-headers-5.10.0-35-common/scripts/Makefile.build:508: /var/lib/dkms/ovpn-dco/0.2.20250718-2+bullseye/build/drivers/net/ovpn-dco] Error 2 make[2]: *** [/usr/src/linux-headers-5.10.0-35-common/Makefile:1864: /var/lib/dkms/ovpn-dco/0.2.20250718-2+bullseye/build] Error 2 make[1]: *** [/usr/src/linux-headers-5.10.0-35-common/Makefile:192: __sub-make] Error 2 make[1]: Leaving directory '/usr/src/linux-headers-5.10.0-35-amd64' make: *** [Makefile:51: all] Error 2 ```