kernel: trelay: fix build warning on 6.12 kernel

kernel: trelay: fix build warning on 6.12 kernel

Make trelay_handle_frame() static to fix the build warning:

/workspaces/openwrt/build_dir/target-x86_64_musl/linux-x86_64/trelay/trelay.c:38:21: error: no previous prototype for 'trelay_handle_frame' [-Werror=missing-prototypes]
   38 | rx_handler_result_t trelay_handle_frame(struct sk_buff **pskb)
      |                     ^~~~~~~~~~~~~~~~~~~

Signed-off-by: Shiji Yang <yangshiji66@outlook.com>
Link: https://github.com/openwrt/openwrt/pull/18637
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>