um: remove "extern" from implementation of sigchld_handler

There is no need to mark the function as extern in the implementation.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202506051226.X8r7X5aa-lkp@intel.com/
Fixes: 8420e08 ("um: Track userspace children dying in SECCOMP mode")
Signed-off-by: Benjamin Berg <benjamin.berg@intel.com>
Link: https://patch.msgid.link/20250605050325.1077208-2-benjamin@sipsolutions.net
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

Author: benzea

arch/um/kernel/irq.c

@@ -691,8 +691,8 @@ void __init init_IRQ(void)
 	os_setup_epoll();
 }
 
-extern void sigchld_handler(int sig, struct siginfo *unused_si,
-			    struct uml_pt_regs *regs, void *mc)
+void sigchld_handler(int sig, struct siginfo *unused_si,
+		     struct uml_pt_regs *regs, void *mc)
 {
 	do_IRQ(SIGCHLD_IRQ, regs);
 }