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 <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/18637
Signed-off-by: Hauke Mehrtens <[email protected]>

package/kernel/trelay/src/trelay.c

@@ -35,7 +35,7 @@ struct trelay {
 	char name[];
 };
 
-rx_handler_result_t trelay_handle_frame(struct sk_buff **pskb)
+static rx_handler_result_t trelay_handle_frame(struct sk_buff **pskb)
 {
 	struct net_device *dev;
 	struct sk_buff *skb = *pskb;