kernel: fix build of kmod-udptunnel4 and kmod-udptunnel6

kernel: fix build of kmod-udptunnel4 and kmod-udptunnel6

This fixes the following errors:
```
make[4]: Entering directory '/home/runner/work/OP/OP/openwrt/build_dir/target-aarch64_cortex-a53_musl/linux-mediatek_filogic/linux-6.12.57'
  MODPOST /home/runner/work/OP/OP/openwrt/build_dir/target-aarch64_cortex-a53_musl/linux-mediatek_filogic/ovpn-dco-0.2.20250801/drivers/net/ovpn-dco/Module.symvers
ERROR: modpost: "udp_tunnel6_xmit_skb" [/home/runner/work/OP/OP/openwrt/build_dir/target-aarch64_cortex-a53_musl/linux-mediatek_filogic/ovpn-dco-0.2.20250801/drivers/net/ovpn-dco/ovpn-dco-v2.ko] undefined!
ERROR: modpost: "setup_udp_tunnel_sock" [/home/runner/work/OP/OP/openwrt/build_dir/target-aarch64_cortex-a53_musl/linux-mediatek_filogic/ovpn-dco-0.2.20250801/drivers/net/ovpn-dco/ovpn-dco-v2.ko] undefined!
ERROR: modpost: "udp_tunnel_xmit_skb" [/home/runner/work/OP/OP/openwrt/build_dir/target-aarch64_cortex-a53_musl/linux-mediatek_filogic/ovpn-dco-0.2.20250801/drivers/net/ovpn-dco/ovpn-dco-v2.ko] undefined!
make[6]: *** [scripts/Makefile.modpost:145: /home/runner/work/OP/OP/openwrt/build_dir/target-aarch64_cortex-a53_musl/linux-mediatek_filogic/ovpn-dco-0.2.20250801/drivers/net/ovpn-dco/Module.symvers] Error 1
make[5]: *** [/home/runner/work/OP/OP/openwrt/build_dir/target-aarch64_cortex-a53_musl/linux-mediatek_filogic/linux-6.12.57/Makefile:1897: modpost] Error 2
make[4]: *** [Makefile:224: __sub-make] Error 2
```

ref 1d15a96b29dc
fixes: openwrt/packages#22998

Signed-off-by: Andy Chiang <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/20839
Signed-off-by: Christian Marangi <[email protected]>